3000 People Lived in This Hidden Underground City | Underground Worlds !

Hidden beneath a forest in northern France, they carved their name to make sure we will remember them. A beautiful underground city for 3,000 people. Each room had his family in an ice cave beneath Europe’s largest glacia. Heavenly blue color. How do experts react to the unstable environment to keep visitors safe? They are constantly changing. It’s so dramatic.

And hidden beneath the city of London, how is a state-of-the-art regeneration project improving the lives of Londoners? They don’t believe that there’s a power station right in the middle of the city. Beneath our feet lie extraordinary spaces, caves, and tunnels. The span and the size is just crazy.

 They’ve been designed and built by us. This is the only one with the castle as well as formed by nature. But how were they created? and adapted by who and why. You’ve got to face your fears. Throughout history, subterranean life has captured our imagination. Feel so privileged. We’re going further and deeper to unearth the mysteries, the stories, and the secrets of underground worlds.

Naor is a small village in the Som Valley in the Picod region of northern France. Just east of the village on a wooded limestone hill is a cluster of farm buildings and a small chapel. They disguise the way to a subterranean [Music] labyrinth. For nearly 2,000 years, this tunnel system has played a vital role in the lives of the people of Naor.

There were a lot of invasions just to escape from the enemy they were hiding here. It’s a real treasure. To what extraordinary lengths did the locals go to create a fully functioning underground world on the top of the hill? If they let the smoke out just like that, the enemy could imagine that there was something inside.

 who surprisingly became tourists here in the early 20th century. I can’t imagine a soldier tourist uh during the Great War. And what can state-of-the-art technology reveal about this precious historic time capsule? You can produce a 3D model. They will wear the virtual reality glasses and be able to navigate in the the model. And that’s fantastic.

[Music] The underground city at Nao is one of the most captivating subterranean systems in Northern Europe. It runs 22 m deep and extends for 2 km under the village which bears the same name. In all, there are 28 galleries and 300 chambers in this underground complex. The earliest passages date back to the 2n century AD when the Romans dug down into the hill to quarry limestone.

 Carolene Labju is in charge of this 2,000-year-old wonder. When I first came here a year ago, I did not expected to discover a place like that. It was just amazing, a special feeling. And uh when I discovered the history here, I just uh felt the emotion of the of the site. As we can see, it’s all made by the hand of men. It’s nearly 2 km tunnels here.

 Just amazing. And imagine that in almost 2,000 years since the Romans started their excavations here, the tunnels have slowly expanded and been repurposed as secret shelters for the local people. During times of conflict or invasion, they could have found themselves living and working down here for months at a time.

People from the village uh and they knew that there were a lot of quaries here in this area and they just transformed quaries into those tunnels that was shelter for them. This underground is very well protected. On the top of our head is just forest and thanks to trees we are protected. [Music] For centuries this area of northern France and its people were under constant threat from invading armies.

At that time there were a lot of invasions in this part of France and just to escape from the enemy. They were hiding here themselves in this shelter. The more the locals sheltered here in the middle ages, the more they developed their underground city. They constructed wells, stables, bakeries and chapels in this warren of caves and tunnels.

This is the chapel of the underground and this is exactly like a real church. We know that they were here only to protect themsel and just taking the time to build a chapel inside this shelter is just for me amazing and like crazy. Living underground means cooking food and keeping warm which in turn means lighting fires.

The people who built Na found an ingenious solution to the problem of smoke escaping to the surface and giving away the location of their underground city to their enemies. It seems the smoke ducks were channeled through the chimneys of existing buildings such as the windmill. That was very smart because on the top of the hill they let the smoke out.

 We can see here traces from smoke because at that time they didn’t have electricity. So we can see trace of the history here on the wall. At the height of their use in the Middle Ages, the system of 300 chambers could accommodate almost 3,000 inhabitants. In the room like this, just imagine that on the floor you have stro only stro 12 people could live here.

 a family and uh all the rooms are different. Each room had his family. His family has a door, a key for his own room. In subsequent centuries, the tunnels continue to provide a bolt hole for Na. Evidence of their use continues to be discovered by researchers. Jill Prio, a historian and archaeologist, has uncovered hugely significant artifacts in these tunnels dating back to the 17th century.

 The first time I I enter in this in this cave, it’s uh for archeological research. And during my research I discover piece of potry coins um musket bullets and that prove men was air occupied this this site during the 30 years war. The 30 years war raged in central Europe between 1618 and 1648.

 It was one of the most destructive conflicts in human history. resulting in 8 million fatalities. During this period, the the French kingdom fight with German kingdom and Spanish. Following the 30 years war, this part of Europe gradually became more peaceful and stable. For almost 100 years, these tunnels were forgotten. Then in the late 19th century they were rediscovered.

In 1887 the priest Danikor priest of the village rediscovered the site. He heard about those tunnels those undergrounds in the village and he wanted to rehabilitate the site and to make the site famous to present the site to the world. The abbbert Ernest Denor was determined to revive the forgotten tunnel network of Naor as a tourist attraction.

 When the priest Danur we discovered the site, it was in this room. This is the largest of the chambers in the now complex called Lalv. You are speechless when you’re here because it’s like nearly 30 m high. and you feel just really tiny. I can understand why he wanted to go further and discover the other parts of the underground.

 It was the the work of his life. [Music] The tunnels were a popular tourist attraction for the next 25 years. Even as World War I got underway, a surprising number of visits were made by Allied soldiers seeking restbite from the horrors of battle. I learned the the soldier during the the Great War, they stay on the front line 50% of their time and after they go to the back front to to take a break.

The soldiers would travel to Naur to join the ranks of tourists visiting this underground curiosity. I can’t imagine a soldier during the great war doing do tourist. The farmer knows the caves and they can invite the soldier to discover the caves. Many of the soldiers who came to Naor recorded their names and personal details on the tunnel walls.

 The underground complex contains one of the highest concentrations of historical graffiti of any World War I site. Today we know there is 3,200 name about the Great War. English, Ireland, American, Australian, India, um, Canadian, French. Um, it’s a real treasure. Sometimes I carve their name to make sure we will remember them. It was just a proof that they were here to fight.

The men wrote the name on the wall. Put a piece of them on the wall. Today it’s it’s a perhaps it’s my last day because tomorrow I I return on the front line. One of the names Gil found and researched is that of British soldier Corporal Samuel Mikosha. He was on the trenches and the German shells fall on the trenches.

 Uh, lot of of his comrades died, was wounded and some of them was covered by the soil. With his commanding officers dead, Corporal Mikosha gathered his surviving comrades. He said, “Come with me.” And with their end they dig out four soldier. Um they and he save this for soldier. Samuel Mikusha survived the first world war. He was honored for his bravery and awarded the Victoria Cross.

It’s very famous for me because it’s a real hero but a simple man. [Music] With so much graffiti as a record of the experiences of many thousands of Allied soldiers, the tunnel system is highly significant to historians. Over the course of 2 years, a cultural heritage team of four spent 20 days scanning Naor and then building a walkthrough 3D map of the labyrinth.

Many of their projects involve the scanning of wartime momentos and Pierre Gusare is their team leader. We are recording this uh site in order to to archive everything. You can produce a 3D model and after that make some 3D printing or some replicas. So that’s also things we can do after the recording.

 The team’s efforts to accurately record the tunnel system are due to the site’s historical significance, not least because of the first world war. This uh place uh tell us the story of the great war. So um I think it’s very important to uh keep this information forever. To do that we have to work very accurately because we merge several kind of data.

 They will wear the virtual reality glasses and be able to navigate in the the model and that’s fantastic sharing of technology and the local history. We are starting to to scan there with the terrestrial laser scanner and uh you may see all around the side this uh white spheres. Each sphere will have very accurate coordinates and that will help to merge everything together in the best way.

Mapping the extensive network at Naor has been a vast and painstaking project. Each of the 300 chambers has so much detail to be recorded as a digital scan dating back to the site’s origins as a Roman limestone quarry. Today we are mapping this underground crow and uh it’s first time we we scan this area.

 This underground core is very interesting for us because there there is a huge volume. we can better see faults and geological structures. So, it’s very interesting for us uh geologists uh to to make a map in this area. For more than 2,000 years, the underground city of Noor has been at the heart of this community. Those who look after it today are determined to make sure it continues to remain that way.

I’m very proud to my little job, my little research uh with a signature. We have to preserve it. It’s uh it’s beautiful. So, we have to continue the work. Nowhere else you can find uh an underground world like this. [Music] Iceland is an island country 40 km south of the Arctic Circle where the North Atlantic and Arctic oceans meet.

 Iceland has Europe’s largest glacia. But how has this 8,000 square kilometer mass of ice created some of the most stunning subterranean realms on the planet? Ice caves are alive and they are constantly changing, evolving. It’s so dramatic. Why are these glittering ceilings important to the scientific study of the earth? We can see inside the the ceiling black lines here and there.

 probably uh many hundreds years old. So we we have history of Iceland here. And what risk does global warming pose for the glacia and its beautiful ice caves? They can disappear very quickly. They are very sensitive to climate. Now the glacia is leed over 100 m per year. Iceland covers an area of 103,000 square kilm. More than 3/4 of its terrain is uninhabited and the island has a subarctic climate of long cold winters and short mild summers.

These are the conditions in which glacias form. And beneath them, carved out by streams of meltwater, are some of the most beautiful underground worlds of ice caves. Professor Magnus Goodmanson is one of the country’s leading experts in the field. Ice caves are formed in all glaciers with flowing water at the bottom.

 The ice tunnels are the pathways of the water that is melted on the surface. They’re formed in summer by the flowing melt water. The ice caves tend to be this sort of heavenly blue colors because it’s light coming from from the sky through the ice. With a whole mass of ice slowly moving above and melt water flowing underneath, ice caves constantly shift position and change size.

 Some can develop to a length of 200 m. Because they are always on the move, it is impossible for a visitor to step into the same cave twice. This is the Treasure Island ice cave, located 7 m deep inside a glacia called Vatna. The ice caves can become quite big in a short period of time.

 Ice cave that’s maybe 2 m high may well form in a few days. Iceland has around 269 named glacias and Vatna is the most impressive taking up 8% of the country’s land mass. A glacier is a body of ice that is large enough and thick enough that it moves under its own weight. Glaciers are very active. They shape the Alps and they make the fjords in in Norway.

 So glaciers are a very strong force of shaping the earth. Vakna. This is about 8,000 square kilometers. By far the biggest glacier in Iceland. The oldest ice we see in Icelandic glaciers is something like 900 2,000 years old. We can see that because there are these really distinct dirt bonds. They are as layers from volcanic eruptions.

 And we have dated these eruptions. So we know how old the ice is. Each glacia can have as many as 30 ice caves beneath it. But around half of those will be too unstable for the public to visit. For centuries, these epic glacias and the shifting ice caves beneath them have fascinated Icelandic explorers. Ana Sigorusan and five generations of his family have become famous in their homeland, exploring their nation’s geology and many ice formations.

 My great-grandfather was in fact the first person to climb the highest peak of Iceland. I guess it’s runs in my DNA. It’s in my blood, you know. I’ve been in ice caves for over 25 years. Just like his ancestors, Ana knows the dangers of ice caves and glacias. During the last 15 years, three people have died while visiting caves, and two of these were because of roof collapses.

Tourists have also lost their lives trying to explore the unforgiving landscape without expert help. Risks include exposure, breathing difficulties from trapped volcanic gases, and falling into crevices hidden by the snow. On occasion, even expert guides have perished. Here, when I’m working across these kind of lowland glacias, you have plenty of cracks and uh you see the denses, so you can go around them.

 But when you are climbing the higher part of the glacia about 1,000 m, you have much more serious kind of crevice risk. They get to be 30 m deep. Some of them maybe it’s snowing up there hiding open crevices and you might be going up cross soft snow across them and you could fall through. For 14 years has been guiding people around this glacia and in 2017 he discovered treasure island ice cave.

First time I was in one of these ice cave was in my early 20ies and uh still remember how it was like coming into a a new world I never seen [Music] before. Water, ice, and light combine to create a magical subterranean environment. In this particular cave we are in now, you know, you you really have so nice details to see this material so blue, you know, that’s amazing. There’s plenty of light here.

Looks like it’s very thin, but uh it’s it’s probably 5 m already thickness of ice here just above us. It’s been like like you’re underneath the sea or something. You can see inside the the ceiling black lines here and there and sand. The sand that is in there for example, this is probably uh many hundreds years old, you know.

 So we we have history of Iceland here in the ceilings here inside the deep cave. These stunning underground spaces constantly evolve with fluctuating seasonal temperatures. As the warmth of summer arrives, ice turns to water. It’s the heat traveling with the river that that kind of melts this into this stormy kind of shape.

 It’s just amazing how beautiful if you’re in the right day, the right time of year, the right right cave for that season, you know, how how beautiful places you can find. You know, this is just the start of the system. We can go actually 200 m into the glacia here. Iceland is definitely the place for icecape exploring. You have so many lowland glacias here and they are so big and easy to access.

Even so, Iceland’s glacias are also retreating. And experts believe it’s due to climate change. The rate of winter snowfall has stopped exceeding the summer melt. This in turn adds to the instability of the ice caves. Guides fear that there could soon be a major roof collapse. There can be as many as 1,000 visitors every day between October and March, so it is vital to make sure they’re safe.

 Stefan Mantel from Iceland’s search and rescue team monitors the constantly changing glacia from above. When you think about where you enter an ice cave, it is almost always from the front of the glacier where the ice is the thinnest and where it is most susceptible to change and to melting away. So the front part of the caves and the roofs there are uh the most dangerous and the ones that we want to really have a very good eye on.

This is a part of the glacier that we visit very often and so even coming here in prior years we saw a lot of change and I found it important to try to document these changes. I’ve now accumulated about a year of of change documented. So to actually map the glacier with the drone, I need to tell the drone to take a lot of pictures from a lot of different angles.

 And then my computer at home combines them essentially to 3D models. So we see really also the shape of the glacier and how that changes. Standing here on the glacier, it looks very permanent to us, but it really is not. It’s almost alive and it changes every day. It’s important for Stefan to regularly monitor how the glacia and its ice caves are changing.

 The last image for example on Google maps of this area is I think four years old and the last image that I have online is 4 days old. It is important to uh document these changes because the glacier we see it not only removes back from the front right it becomes thinner of its entire surface and so if you think about caves for example ice caves the roof is only a certain thickness and so if the glacia changes the roof becomes thinner by monitoring how the ice changes you can see that maybe you recognize okay there is something that needs to be addressed

Stephan’s work with the drone from above of means his search and rescue colleague Solar Sienbian’s daughter can investigate what’s going on below the surface. There’s a lot to think about when you’re visiting ice caves like this. We try to look for things and we have few red flags if we can say so that we’re searching for like do we have cracks in the ceiling? Is it not touching the ground? Is there a lot of water in it? any weather change often just from warm to cold can also h make some tension in the ice. So there are a lot of

indicators that you’re looking for. It’s not just one factor that we can just say it’s you know it’s going to collapse if it goes beyond 10°. It’s not that simple. You know ice caves are alive. We can say almost the glacier is alive. They are constantly changing evolving. It’s hard to describe to people without seeing it, you know, because it’s so dramatic.

 Imagine if you visit a place like this, it will never be the same. Like even if you just visit it tomorrow, it will be a different place. And I think that’s what makes it one of the most amazing places you can visit. The entrance to this giant ice cave is accessed via a 7 m descent down the [Music] glacia.

 Inar brings groups of tourists here. So is meeting solen to see how the subterranean spaces have changed. Here is the true blue I’m always looking for. Is really beautiful. I always find like in the start of each season like in September, October, it’s like going treasure hunting. Yeah, exactly. Yeah. What will you find? Wow. Yeah.

 We can understand why thousands of people want to come here and see all this. It’s changing so fast and so much that you never know what to expect. You take a picture one time and then you come in a week and it’s completely different. That’s the beauty of it, I would say. You know, you’re always finding new things and always exploring. Yeah, the ice caves may not be here forever.

 Since 1995, Iceland’s three biggest glacias have shrunk by more than 250,000 cub m. Any acceleration in the glacial retreat could suggest dire consequences on a global level, such as increased extreme weather events and rising sea levels. One thing that we are seeing in Iceland and that’s a worldwide phenomenon is very fast retreat of the glaciers.

 They can disappear very quickly. They are very sensitive to climate. If things continue as they have with climate change in 100 years there will be no glaciers and no ice caves to to visit. I can’t really imagine my children or grandchildren not being able to experience this like I’m doing today. It’s kind of frightening and sad. It was a ice cave uh I would go into in the year 2010.

 Also another ice cave where the red rocks are and since then now the glacia is like uh 12 or 1300 m up there. It has receded over 100 meters per year. For 1,000 years, the Vadner glacia has been a significant part of Iceland’s landscape. Its people hope they will be able to continue to enjoy and explore this everchanging subterranean environment.

glaciers here in Iceland, they are like climate change barometer and it’s like uh changing so fast. You really have to go for the opportunity to enjoy the underground world of Iceland. The city of London, also known as the square mile, is the center of British finance and one of the world’s most influential business districts.

 Almost half a million people commute into this compact area every working day. These people and their employers have huge demands for energy, heating, and air conditioning. Few realize that power comes from within a fascinating network of tunnels and chambers underneath the city. Some are centuries old. Others were created during World War II in London’s darkest hour.

 This huge area was completely flattened, devastated by the blitz. But what subterranean structures from our past are shaping our future? Welcome to the old engine room. Wow, this space is enormous. And how can 21st century power be distributed around a complex underground network in a city that never sleeps? People are totally oblivious to what’s beneath their [Music] feet.

 On the outskirts of London’s financial district stands a 19th century building that goes largely unnoticed by passers by. The street level facade belies a vast system of underground spaces which house the largest urban combined heat and power system in the UK. At the center of this warrant sits an impressive fourstory energy station and John Armstrong is its director of operations.

 Often when I tell people where I work in London, they don’t believe that there’s a power station right in the middle of the city. And it’s actually only when they walk past they realize what’s here and look look at a bit more detail. So, how did a power plant end up in the middle of the centuries old district of London? The historic Smithfield meat market may provide the answer.

 When the building was redesigned in 1860, it gave rise to a system of underground spaces which have been integral to its growth and expansion over the years. Roger Morgan, an expert in subterranean, has been investigating the legacy of the changing uses underneath these buildings. We’re standing in Smithfields, which is the medieval meat market.

 Below the the main selling floor was a huge railway goods depot where the meat trains from the north would discharge their cargos and it would be hoisted up onto the selling floor on hydraulic lifts. And then next door is a coal store for storing the meat which wasn’t sold immediately. Meat was stored deep below street level under controlled conditions.

Carcasses, generally cows from the market over the road, were bought in and kept cold in this building. So, this room is 20 m long, 10 m high, 10 m wide, so 2,000 m cubed. Potentially, you could fit about eight double-decker buses in here. So, this is original steel work holding up the structure from the Cold Store from 1895.

In the late 19th century, as electricity and refrigeration developed, coal powered boilers were installed below the cold storage room. Here they generated much needed power for the Smithfield market. In this vault over here, this vated structure, which is all bricklined, you can see some of that history from the original coal power station that was here.

 And you can actually touch the walls that in 1895 they burnt coal in this space. It’s absolutely amazing to think that over 100 years ago, people doing the same job that I do now were here. Very, very differently, but generating power and distributing it around the city. During World War II, the city of London was heavily bombed.

 The last of Germany’s huge rocket propelled missiles hit Smithfield in March 1945, killing 110 people and injuring dozens more. London’s authorities made plans to rebuild the city and especially make use of its underground spaces. Roger is looking at a 1940s map which shows all the plots ideal for redevelopment.

 So in orange is the area devastated by the blitz and the V1’s and the V2s. The whole area was completely flattened which gave the opportunity to the city of London to redevelop the area comprehensively and provide much more service subways to serve the Barbacan estate and other buildings. So nearly every modern building since the 60s in city of London will have a huge extensive basement.

 So the potential for development for the network in the future is obviously highly significant. The existence of these postwar spaces, some three stories deep, proved to be an asset when the Cold Store closed its doors as a refrigeration plant in the 1970s. It was to be turned into a more advanced power station using two 350 ton ship diesel engines.

A system was built to supply power, heat, and cool water for air conditioning that weaves its way through the subterranean heart of London’s financial district. The use of existing spaces saved on digging new tunnels. Kish Barrett manages this subterranean power network. You wouldn’t dream of putting a power station in the middle of a city.

 However, the use of subways and underground locations where people are are totally oblivious to what’s beneath their feet is a clever way of delivering local energy. We could only really do this given the access to subways, tunnels, and car park locations. Otherwise, we would have to be doing civil excavations in major roads. More than 6 km of heating pipes and nearly 5 km of cooling pipes deliver vital energy supplies to various residential, business and public buildings.

 Allows us to deliver the refrigeration at the Smithfield Meat Market and also for the daily operation of the Guild Hall which is the city of London’s corporate center. The Cityen underground complex is not open to the public but Kish has offered Roger a special look behind the scenes. I’m quite interested in getting into the building which apparently is extraordinary.

This guided tour fulfills Roger’s ambition to see how this historic old power station So this is where we begin our descent. has a new use well into the 21st century. Welcome now to the old engine room. Oh, thank you. Wow, this space is enormous. So this is where the old engines were cited and you can see the the the vastness of the room really gives you a feel of how big that equipment was.

Yeah. If you just look up is that is that soot of that? It is. It is. It’s the exhaust from that engine process. Yeah. In 2016, the two diesel engines were removed to make way for two brand new low emission 4.3 megawatt gas powered engines. The refit cost £26 million. These new efficient engines mean that the city gen complex can now supply tens of thousands of megawatt hours of heat, electricity, and cooled water to more than 11,000 homes and businesses in the city of London.

 It’s all part of London’s aspiration to be carbon neutral by 2030. When we look at the technology now, the higher levels of efficiency, how quiet they run and also how we use technology and digitalization, things have come on a long, long way. This power station is known as a combined cooling, heat and power plant or CHP.

 These new gas powered engines are 80% more efficient than conventional power stations that burn fossil fuels and are lower in pollution emissions. The system has the ability to capture and reuse excess heat at every stage of the energy production process to keep down fuel use. It’s all monitored from a central control room by Le Oloule.

 This is central London’s biggest trigeneration facility. Trigeneration is the ability to create heat, chill and electricity from the same source. Up here we optimize the production of heat and chill which ensures that the CHP is more efficient than a normal gas engine. We store the excess heat in the thermal stores. The thermal store is like a giant central heating hot water tank.

 This is where we generally store our production of heat. It’s full of hot water. First time I walked in here looking up and seeing the uh the sheer size, it hits you how big the energy center is. The volume of the thermal store is 320 m cubed. Takes about 2 and 1/2 hours to fill it. This is the beginning of the thermal journey where the hot water from the thermal store goes into the pipe and then goes onto the network.

 Even as the hot water completes a circuit of the network, it remains at a high temperature. Much less energy is needed to bring it back to the boil than to heat it from cold. The whole tri power system runs a total of 10 km. Manager Kish is now showing subterranean expert Roger how it flows out through the existing subterranean spaces underneath the financial district.

 So Roger, now we’re going down into the basement areas of the meat market. Okay, we’re probably about 250 m from the power plant. Okay, the pipes are running along the length of the underground car park. It’s this sort of area or opportunity that we look for to get energy from point A to point B such that we’re not having to do excavations in the road.

Yes. Which is an interesting feature of the whole system. So what we’ve got here is four sets of pipes. Uh two sets are the flow and the return of the heating system and then there’s the flow and return of the chill system. So the whole thing’s a bit like a domestic central heating system only 200 times bigger. Absolutely.

 the flow in return. Exploring this space, I would never know what these pipes are. So, it’s fascinating to find out. One of the beneficiaries of the energy system is the Barbcin estate built between the 1960s and the 1980s with its mixed use of private homes and a public arts complex. The city genen pipe network runs right under it.

Here’s the run of the pipes coming through the building. And again, this is allowing us to use underground space, not only to serve the energy requirements for the Barbanet Center itself, but also take that energy beyond the Barbinet Center to other buildings along the network. The furthest destination on the energy chain is London’s Guild Hall.

 Dating back to the 15th century, it was badly bombed during World War II. Reconstruction allowed the addition of new basement levels. So, I’m deep under the guild hall. Roger has now reached the final stage of his underground tour. According to my research, we’re three stories down, 10 m, and it’s the deepest part of the Guild Hall.

 Where we were standing is the southern fringe of the Blitz bombing. So, to the north of us, there was complete devastation. There was just rubble at the end of the war. So there was a huge potential for the whole area and incorporate service subways between all the buildings which advantages city now in that they can now lay their pipes through them.

Even though the energy has flowed through a warren of tunnels, it’s clear from the monitors how efficient the tri energy system is right up to the extremities. Oh yes, it’s showing the temperatures of the water going in and out. So the in temperature is 93.7 and the out temperature is 71.3.

 So the water is coming from the the the hot water store at Citigen is reduced by 20° and is returned to Citigen and then they only have to boost it by 20° rather than reheating it from scratch. As our cities expand and our needs for cleaner, more efficient energy grow, innovators strive to meet London’s 2030 target to become a carbon neutral city.

Isn’t it fantastic that these underground worlds are supplying lowcarbon power to the city of London’s iconic buildings? What’s underground? the subway tunnels, the car parks and basement allows us to install a network in those locations underground that’s benefiting the air quality above ground. [Music] Beneath a tiny village, a labyrinth of secret tunnels. Oh my goodness.

 Why were they so important during one of the world’s bloodiest battles? This is the biggest single concentration of graffiti on the soul. What spectacular uses have been found for these Welsh caverns? Amazing just to think that everything was actually mined here by hand. Carved out by slate miners in the toughest conditions. They are razor sharp.

 They would cut you in half. And deep beneath a glacia. It’s the most volcanically active spot on Earth. What stunning spaces lead to the entrance to Earth’s fierce and fiery core? Beneath our feet lie extraordinary spaces, caves, and tunnels. The span and the size is just crazy. They’ve been designed and built by us.

 This is the only one with a castle as well as formed by nature. But how were they created and adapted? By who and why? You’ve got to face your fears. Throughout history, subterranean life has captured our imagination. Feel so privileged. We’re going further and deeper to unearth the mysteries, the stories, and the secrets of underground worlds.

 France is the largest country in Western Europe. Its northern region has an exposed landscape of farms and villages. 12 m below this picturesque village lies a complex of tunnels carved from the soft chalk bedrock. These chambers were to play a key role in one of the bloodiest battles in human history. It’s a real privilege to come down and experience this.

 It’s one of those places where time stands still. What unbelievable discoveries have been made in these depths? It’s unique. It’s It’s a unique piece of graffiti. How did the tunnels play a crucial role during the First World War? They actually had the toughest job to do. They had the toughest job. Yes.

 And what can be learned from these thousands of handcarved messages from the past? He was one of the first black professional footballers. This place is a time capsule. [Music] 160 km north of Paris, Bancor is a village of around 500 people. The church of Santonor dominates the surrounding countryside and within it lies an entrance to a secret world.

 These tunnels contain mysteries from many different historical eras. Battlefield expert Vic Payak has been researching the clues left behind. The first thing you actually see uh here is obviously a man-made structure, bricks, but then you enter another world altogether. The villagers of Bancor started digging here in the 15th century.

 They would have found the poorest chalky terrain easy to excavate by hand because it drains well and requires no shoring up. The tunnels were built for two reasons. They would actually come down here for safety, for security, or to see out a particularly bad winter. Come down below here because it’s the same temperature all year around.

 It might be absolutely freezing up above, but here it would be actually more hospitable. Construction started relatively shortly after the Hundred Years War when there had been lots of battles here coming backwards and forwards. That was in 1453 when the French finally won a centurylong fight with the English for the right to rule their own country.

 But there would be many more wars in the coming centuries. These tunnels gave the villagers a secret hideway to take refuge from all the fighting. Incredibly, the entire village would disappear under underground bringing with them their cattle, their sheep, their pigs, and they would hunker down here. These were secret places. As far as the enemy were concerned, they’d find a deserted village.

 The entrances would have been hidden. Armies that passed through this wouldn’t have had the slightest idea. The villagers dug out tunnels and 50 separate rooms to provide accommodation. The largest of them covers 30 square meters. The rooms were actually used by individual families in times of crisis and there would have been a wooden door here and you turn the lock on it and uh you know cuz it was your property and you can actually see although the wood’s long gone, you can still actually see the evidence of where the door frame

[Music] was. This was the environment for the family. They would have actually lived in here and had their uh bits and pieces here. But actually this was for the cattle and these are actually the stalls for the pigs, the sheep, whatever beasts they actually had down here. Vic is meeting local historian Jean Luvian who has traced his own family back through time by analyzing the carvings on the tunnel walls.

Super. Okay, that was actually his grandfather and his two great uncles. John Luke comes from a long-standing family. They’ve been here for for centuries. There’s a long connection here. The most extraordinary carving can be found at the entrance to one of the underground chambers. Right.

 This is the oldest oldest inscription here in the uh in the shelters. 1711. He did his family tree some years ago and to his absolute amazement and delight of course uh he found the name and the correct time scale. Members of his family all those generations ago slept in this cave. It is unique. It’s it’s a unique piece of graffiti that actually directly connects one of the rooms to to to a family in the village.

 In the year 1711, Europe endured one of the coldest winters on record. It’s fascinating to see visual evidence of a family taking refuge from the elements. Almost without a shadow of a doubt, the entire family, indeed the entire village came down into these caves to see out that awful period. He says it’s a very important place for the community here in Busanor because it’s a it’s a place that bears witness.

The tunnels played their biggest role in Boozenor’s history when the tiny village found itself on the front line during World War I. It was a war between two power blocks, one headed by Britain and France, the other by Germany that lasted from 1914 to 1918. Anor is close to the site of one of its deadliest battles along the course of the river Som.

 French troops are actually in occupation here. The song actually was quite weird until the battle of the S in 1916 when the British arrived. The villagers of Busanor fled, leaving the British army in the town. During the atrocious battle of the Som, soldiers fighting in muddy diseaseinfested trenches found these secret tunnels, like others before them, as a welcome refuge.

So to be 12 m underground in structures such as this was was heavensent for for the British military. [Music] Immediately what you see here is the life that the men actually lived uh when they were down here. Soldiers did everything down here. It seems that this could actually have been the chapel.

 There at the back you the earth weird jar is actually used for rum ration. The soldiers actually got a daily tot of rum and it would have been very welcome to keep out the cold. The Battle of the Som lasted 5 months from July to November 1916. On the first day, more than 19,000 British soldiers were killed by shelling and machine gun fire, the largest loss of life suffered by the British army in a single day.

What you can lose sight of in the Great War, it’s individuals. The tunnels provided those men with shelter and relief from the carnage as well as an outlet for their graffiti, an expression of their hopes and fears carved into the soft walls. We just see the names and the marks they’ve left in history.

 These were real people. Seems a bit of a cliche to say, but of course they were real people. These men all mattered to their wives and their loved ones. And you can lose sight of that, but you don’t when you’re in a place like this because you can see their names. This is the biggest single concentration of graffiti on the song. This is remarkable.

 A veteran uh of the Great War said it was 90% sheer boredom, 10% sheer terror. For these men, it could be here today and gone tomorrow. This was a maybe a final chance to to leave your mark for posterity. There are more than 1,500 signatures and drawings in these tunnels that date from the First World War.

 Some were carved by American and Canadian soldiers billeted here as part of the Allied Force. Huge variety of soldiers here. There’s actually uh two brothers here side by side here. Look, you can see this EA Jarrett and SHR Jarrett. Canadians both in the signal, can’t they? Yeah. 12th Brigade where the soldiers have given their regimental numbers.

 Historians have been able to trace their military careers both during and after the war. Ray Cleansing, this guy was from North Dakota. He was to win two military medals for bravery. So he survived his encounter with the Battle of the S. He actually died in 1981. Another carving refers to a particular British regiment.

 One of its officers was a celebrity at the time. Lovely regimental badge here of the 17th Middle Sex. The Die Hards was their nicknames. They were the unit that had certain Walter Tull was a pre-war footballer for Tottenham Hotspur and uh Northampton Town. He was the one of the first black professional footballers um and certainly the first black British officer.

 He was actually to die not far from here in 1918. 3 million soldiers fought in the battle of the sum above these very caves and 1 million were wounded or killed making it one of the bloodiest battles in human history. During a bombardment the church of St. Honoray was destroyed. It was rebuilt in 1920 in the shape of an artillery shell as a tribute to the fallen.

Richard Stenning from Southwest England recently discovered that his greatgrandfather, Major General William Ryfraftoft, was among the British forces here 113 years ago. It’s intriguing to come here and uh retrace some of his steps. Richard’s on his way to meet Vic to explore the tunnels where his greatgrandfather once took shelter.

 I knew nothing about the underground space at all. Does sound an incredible place. I’m excited to see what’s there. Oh my goodness. Look at all that brick work. [Music] Okay, Richard. So, we’re dropping into the tunnels here at at Boozen Core. Have you ever seen anything like this before? Nothing quite like this. No, it’s not built for creature comforts.

 As you can see, you’re stooping, banging your head would have been pretty common place here. How many would have been here? Would they have been packed in? They would have been packed in pretty solidly. Yes. Uh they were safe down here and away from the bombardments, which were pretty continuous.

 And you’d have still felt it. The war made a devastating impression on an entire generation. Present at the battle of the psalm was a 24 year old officer in the Lancasher fuseliers who would later become a phenomenally influential storyteller. There’s some anecdotal evidence that uh Tolken was actually I saw something there that he he served around.

 Yeah, he certainly served here. That’s that’s not open to debate. The famous Lord of the Rings author JRR Tolken undoubtedly drew on his wartime experiences in his epic saga of good versus evil. Underground worlds play a crucial role in the fantasy land he created named Middle Earth. From the Hobbit’s burrows to the minds of Moria and the dwarven realms, he could very well have been in these in these tunnels.

 It would have been very cool to have left his market here. That was something that would have been something. Yes. Then some thought to him for Middle Earth. As a major general, Richard’s great grandfather, Bill, would have commanded a division of around 18 to 20,000 men. There were 50 British divisions at the Battle of the S. What do you know of your great-grandfather? It’s only recently in finding his diaries and things that I’ve really looked at his history and realizing it was a divisional commander in this important part of the in the

first part of the psalm and the probably some of the heaviest fighting is certainly bringing it to bringing it to life from being just a fact of one’s past voyage of discovery to see what he went through. Yes. Unlike later conflicts during World War I, senior officers still found themselves in mortal danger on the front line.

 There is a All right. I’ve actually never seen the photo of him. So there you go. Yeah. Tremendous. And this is uh volume three, Confidential Diary of the 32nd. That’s the original, is it? Yeah. Goodness gracious. Yeah. It’s very rare to uh to see something like this. William Reich’s diary of the first day of the battle shows the risks he took with his men.

While around them, vast numbers of British and Allied lives were lost. Rode with Lieutenant Colonel Deuzanor as we alone were actually to break through the enemy main position on the first day of operations. They actually have the toughest job to do. They have the toughest job. Yes. Vic has a surprise for Richard.

 Let’s have a look. The name of his greatgrandfather is carved into the rock face. Ah. Ah, there we go. That is it. Gosh, Bill stood here right in this spot. The graffiti is always from other ranks and you know to actually see major general in charge of 20,000 men. It was absolutely he stopped me in my tracks. It was fantastic.

32nd division 1916. Quite extraordinary really. It does bring love to your throat. Yeah, that’s over 100 years, isn’t it? It will take a bit to sink in with that really being the officer commanding. You’ve got these lives of tens of thousands of men as your responsibility. How would I have cope with that? Could I have stood in his position and done that? It makes it all real.

 Major operations at the Battle of the Psalm ended on the 18th of November 1916. Visiting these caves has brought Richard one step closer to his greatgrandfather. It’s a real privilege to come down and experience this, to have this opportunity to see where my great-grandfather might have been. Um, to tread perhaps where he trod. The walls serve as a lasting memory to Allied soldiers who stood firm in the fight against military aggression.

 But these caves have offered sanctuary for centuries, and perhaps they will do so for hundreds of years to come. This place is an awful lot more than just my great-grandfather. Think of how many people were down here, the soldiers during the war for the local inhabitants all through the years.

 It’s one of those places where time stands still. Wales, a country with a land mass of 20,000 km and a population of 3 million. It’s mostly mountainous, especially in its northwestern [Music] region. Beneath these peaks lie a series of mines carved out over hundreds of years by generations of men and even children. Now a new underground world is making use of the caverns they left behind.

This is the slate capital of the world. What conditions did the workforce endure as they mined this so-called gray gold? What these men went through, the conditions they worked under, they risked their lives every day. And how have the mine workings been transformed by adrenaline junkies? It’s like pictorium times engineering mixed in with modern engineering.

 The hills around Bla Finino in Gwynneth County are strewn with rocky waste material left over from a mining process that defined the local community for centuries. Best known for its use in roofing and flooring, slate was first mined here by the Romans around the year 45 AD. But it wasn’t until the industrial revolution in the mid 19th century that the demand for this precious rock hit its peak.

 This mine was opened by a gentleman by the name of John Whitehead Graves. John came here in 1846 with his team of men. They were here for 2 and 1/2 years and found nothing. Just before the end of the third year in 1849, they struck the old vein of slate and he never looked back. The mine extends 1 km under the mountainside and contains more than 25 km of tunnels and 250 chambers spread over 16 levels.

 They’re accessed by a narrow gauge railway, the steepest of its kind in the UK. The mine closed down in the early [Music] 1970s. Brian Jones once worked in the mine on the maintenance team and is now a guide. This is a perfect example of the way the slate runs. The veins run on a 33° angle. The world famous slate vein was formed 500 million years ago when this area of Wales was beneath an ancient seabed.

 A massive volcanic eruption put huge amounts of heat and pressure on the seabed, pushing it up to an angle of 33° and baking it solid in the process. We’re down now approximately 400 ft underground. The deeper you go, the better the quality of the slate. It’s all to do with more pressure it’s had and it’s had more volcanic heat.

 During the slate mining boom of the 1880s, the population of Blaine Fastenio rose from around 3,400 to more than 11,000. Was one of several mines in the town and miners flooded in to work this rich seam. Now, this looks to me like good quality slate. Look at the very fine graining in in the slate itself. Uh you can see here where there’s been a previous minor drilling and you can see the blackness in here where the uh the gunpow where the blast has been.

 By candle light, men would use basic hand tools called jumpers to work their way into the rock before setting an explosive charge to blow large pieces from its foundations. That’s the natural crack in the rock. A rockman would drill in here maybe down about this this depth, charge it, and blow it out. When he blows this out, then hopefully more than likely would it would break here and then this nice lump of slate would fall down here.

 Up to about 1880, 1890, this would have been done by a jumper manually. By the end of the 19th century, at the industry’s peak and coinciding with the explosion in Victorian house building, half a million tons of top quality slate was extracted every year. This was enough to produce tiles to cover an estimated 14 million square meters of roof.

 Local man Phil Jones can trace his family connection with this mine back through three generations. I remember coming here with my father and my mother. My father always used to say down over there is where he worked with uh his father uh my grandfather in the ‘ 70s. Later on in life, I found myself in the same place. It’s a privilege to be able to work here where they worked and to uh pick up on the history of this place because it’s a big part of everybody’s lives if they if you come from Blafest.

Today, Phil works in the mine as a guide retracing his ancestors footsteps. So, yeah, this is where my father and my grandfather worked. My father was my grandfather’s apprentice, but rockmen is what they were. and they would be extracting the slate. In 1912, drills using compressed air were introduced to replace the primitive hand tools.

 This is uh one of the drills in Janva. They call this a small engine. You’d have a pipe for the compressed air on the side. You’d have a drill bit at the bottom. And this is what my father, my grandfather would have used. It weighs about 40 lb. It’s very heavy. Even though modern tools were brought in to help excavate more slate, they continued to use tried and tested materials to suspend themselves high up on the rock face.

 The reason why they used chains was cuz um rope would actually rot in this in these conditions eventually. The men, they had to pay for the material in the early days. And what they would do is they’d fasten the chains to pegs like this with fuse wire. And then they’d like the fuse wire and they could get the chains back.

 So they looked after their tools. Later on when the company paid for the tools, they just left these chains in place. The skillful miners would scale the wall on chains. Once the men were in place, the chains would be secured around their legs. So that locks into position and it frees my hands and then I can work on the rock surface.

Just move my leg like that and I’m out of the situation fast. Don’t think I could do that job. The chambers were intense spaces to work. It could take a group of four people up to 20 years to excavate a chamber this size. There’s about 10 chains here. Now, there would have been a man on each of these chains.

 So, this this rock face would have been quite a busy rock face, and they would have been up there for uh maybe 10 hours. It was a dangerous place to work. Life expectancy of a minor was just 45 years old. Now, you could be working down in this chamber here, looking up at these dangerous pieces here, hanging over your head all day.

 When these pieces go, they are razor sharp. They would cut you in half. In between miner shifts, workers known as dangermen were employed at night to remove unstable rocks, helping to make mining less hazardous. They would have been working on the tops of these ladders. Wrap your leg around the top rung. Hang on for dear life.

 Crowbar in one hand, lantern in the other, levering and banging away at these pieces here, getting these dangerous pieces down from here, making this place safer for the next day. Now, they weren’t paid much for this job, even though they risked their lives every day. A lot of respect from the men and a lot of respect from the the management for doing this.

 Richard Roberts began working in the slate mine industry when he was 15 years old. There you go. Oh, it’s probably the ideal size of a roofing slate. This is where they made the wages. These men up here actually made money. The men in the ground didn’t have such. When slate mining began here, it produced 90% waste, which can be seen strewn all over the hillsides in North Wales.

 But today, with better machinery, waste is only 10%. There you go. This is your finished product. And this is what these guys were aiming for. Producing roofing slates so they could make a wage. These places should never be forgotten. Because of all the hard work and the suffering that went on into places like this to create this to keep us all dry, the finished slate would find its way to the docks in Liverpool before being shipped all over the world.

It’s important that everybody remembers the people that came here, the conditions they worked under. It must have been traumatic for these young boys of 80 years old working down here in the cold and the damp when the life expectancy was 45. These places should be kept open as long as possible. The people can reflect when they come down here what these men went through every day just to put some food on the table.

When part of the mine was reopened in 1972 for tourism, visitors were able to follow in the footsteps of the men who shaped this countryside. In more recent years, the unique environment of the Slate Caverns has become the perfect home for an award-winning cheese in an area of the mine that is not open to the public.

 We bring the cheese down here when it’s at 11 month old and it’s stacked on the shelves here for 3 months and then carried back up to the crearyy to be cut and sold. Sean Jones, a cheese quality manager from South Canavan crearies, understands how important it is to find the right place to store 50 tons of product. Bringing the cheese down here with the atmosphere and temperature and the pressure down here gives a unique intense flavor to it.

500 ft below the surface. The first room they chose to store the cheese was excavated in 1856. Because of the pressure down here, that makes a difference to the way the texture and the flavor of cheese changes. If temperature was too low, then the cheese wouldn’t move on. It wouldn’t mature go through the maturing process.

 So that’s the ideal temperature, 7 to 8° for the maturing process to move on. So this is extra mature cheddar now, ready to go back to the creamies. So, what I’ll be doing now is checking this cheese for the flavor and the profile before we take it back. This stuff’s good to go. A ton of cheese is brought up and down on the vertical train each day.

 This is a hard job where we have to carry the cheese up to the train ready to go back to the creies. It seems only fitting the cheese is stored in these old caverns as it would have been part of the miner’s staple diet. Slate mine has been at the heart of this region of North Wales for centuries. In 2014, the world’s first underground trampoline park opened inside the caverns.

[Music] We employed a team of French fishermen who designed the net system and then the zip wheel construction team were brought in to to install all the anchors. No machinery uh plant crane scaffolding was used. Uh everything was done by hand. As the miners before them, engineers worked by hand fixing anchors to suspend giant net trampolines, slides and tunnels.

 The netting is is basically fishing nets. The bounce actually comes from the tensioning system and the the belay cables that actually connect the the main net floor to the to the walls themselves. It’s the only one of its kind in the world, combining modern adrenaline sports with history. People come from all over the world to play on this enormous underground net adventure.

[Music] It’s like pictorium times engineering mixed in with modern engineering. I just love the way that we kind of mix the modern technology with the historical side of things. The first time I came down, I was just blown away by the sheer size of the chambers. Just to think that everything was actually mined here by hand and taken out by hand is just it’s just amazing.

 The way I describe um the underground cabins here is just truly unique and um pretty mind-b blown really. Throughout its history, people risked their lives to excavate huge caverns for this precious slate. Now, these intriguing subterranean spaces have been given new life. This was a solid piece of rock at one point and the men have carved all of this out uh by hand, blood, sweat, and tears. And it’s very humbling for me.

They put food on the table. I don’t know where I would be today uh without them doing what they did for the community, for the town, for the families, and that’s what it’s all about. [Music] Iceland, located just outside the Arctic Circle, known as the land of fire and ice. 11% of this island is covered by glacia.

 But beneath the frozen surface lies a fiery volcanic heart. This is what constructed Iceland. It all came from here. As you go deeper in, it becomes more and more spectacular. What extraordinary subterranean phenomenon continues to amaze the most experienced of geologists? What do we have here? Wow. It’s just like a river system.

 Difference is it’s in the underworld. 40 km from Iceland’s capital, Reuvic is one of Iceland’s most spectacular underground worlds. Thor Thordes, a professor in geology, has been studying this area of Iceland for over 50 years. First time I came here is 1965. I came here with my parents. First time I saw this, it’s just simple. Wow.

 It was amazing. The lava tunnel known as Ruvol Hadla is more than,300 m in length. In Icelandic, the name loosely translates as the cave on the hill with the holes. It’s a fantastic place of volcanology. Absolutely fantastic. It has everything. The tunnel is actually a lava tube, a void left beneath a hardened layer of volcanic rock when the molten material has drained out underneath.

 This is one of the largest of 500 lava tubes on the island. One of the things that always impressed me here is these beautiful skylights where you can actually see up into the sky. Ruler’s Hadlia was formed in this lava field more than 5,000 years ago. This is the lava that actually constructed the tunnel over the and it just walked across the landscape gradually.

 As the lava came out of the narrow tunnel and these are all what we call loes and they broke out. With each new eruption, surges of lava forced their way underground through the cooling skin of rock. One giant blob of lava after another. As the tunnel extended in length, it resembled a series of domes. The lava to lengthen step by step and gradually moving forward.

 But then at some point the pressure in the lobe exceeds the strength of the crust or the envelope of the lobe and lava breaks through it and it breaks through a fairly narrow point and then it spreads out and then you repeat this process. Narrow point spreads out. So the chambers represent the lobe, the shoots represent the point of breakout.

As the active lava traveled, it left behind its story in its hard crust. Earth’s history is written into rocks. All we got to do learn how to read the rocks. While the river of lava continued to flow inside, a 10 m high ceiling formed above. As it cooled, it solidified creating a network of colossal chambers up to 30 m wide.

 So here we have the inner wall of the original inner wall of the lava tunnel and you can see this black shiny surface. So you had a viscous lava here which is exposed to heat and it starts to run down. It gets heated up partially melted and is running down from the this little overhang here.

 and ice here, lava here. Interesting interplay between these two phenomenas formed by both formed by liquids. One which is at 0° centigrade and the other one which is at between 1100 and 1200° centigrade. In all there are 14 separate chambers in this lava tunnel that have been created by successive eruptions over many millennia.

Thor is trying to find the place from where these violent volcanic eruptions begin. As you walk through it, you realize you’re walking back in time in terms of lava and placement because the ones which were furthest down when we started, that’s the last load that were formed.

 As we come back this way, we’re getting into older and older parts of the lava flow field. And what we’re seeing is basically how the lava was building this flow field step by step. These are very important constructive forces. This kind of system is the reason why Iceland is here. This is what constructed Iceland.

 Records show that the first exploration took place here in 1909. Since then, the whole system became a source of inspiration to growing numbers of visitors. Among them, Hi Christensen, one of Iceland’s most famous mountaineers and now an expert in extreme environments. When I came here for the first time, I had been to several caves, but this one was by far the the biggest.

But I remember the first time coming here seeing skylights like these, and not just one of them, but like three of them. And that’s very very unique. Over a century, the constant footfall of visitors led to damage. In 2016, the tunnel was closed to the public while Hi and his team took on the task of installing environmentally friendly infrastructure to preserve the precious caves.

It took us a while to figure out what would be the best material to work on where we need to build platforms. We could have put wood, a platform made of wood, but it doesn’t fit in here. It doesn’t feel natural. And although steel doesn’t either, a rush steel gets as close as possible being natural. Also, everything we used, all the bolts, they will not uh penetrate any chemical into the environment.

 Any wires used in here are special wires that will not leave anything behind. So the idea was always that we can go here in 30 years, take everything out and the cave will be exactly like it was before. Creating low impact access for visitors to get deeper into the subterranean world is important. But without the natural light of the outer chambers, experts needed to find a way for people to safely explore.

People don’t realize how pitch black it is inside the lava cave because the the light nor the sound travels anywhere. So just a few meters further in here, if we would turn off the light, we’ll have 100% complete darkness and the eyes start to play trick on you. You start to see white spots on the corner of your eye.

 It’s the eyes trying to find something. If you’re in here for, let’s say, 2 months, you develop bl cave blindness and you become blind. you actually become blind. Your eyes will adjust back once you’re out in the daylight, but part of your eyesight will get ruined forever. While the lava tunnel is a remarkable creation of nature, its wonders are enhanced by a complex system of cave lights designed to work in harmony with the natural environment.

Agustolson was given the task of creating this design. So our project was to get the right colors. It was not supposed to be like uh a Disney project. It was really important uh from the start that we could we was just trying to hide everything. Cables, lamps, comfort system. We installed about uh 100 lights here.

 Uh they’re all of course waterproof because of the ice and falling ice, rain, and so on. The lamps can work down to minus 35 and up to plus 30. This is definitely one of the wow factor here because it’s getting through here and just seeing this particular colors in the ceiling. It’s so many colors here. It’s fantastic. Agugust was given a very specific lighting brief by Halley to bring to life the iron, phosphorus, potassium, and other vibrant minerals deposited by awesome volcanic power.

 We don’t use colors in the lights. We don’t need that because we have all the colors in the world in the formations of the lava. I was really surprised how colorful it was. It was just perfect. It’s beautiful. The whole thing comes to life. you get this kind of wow factor. You have to use your imagination to realize how much heat, how much power is in here, that it’s creating rock as it as it flows. It’s a flowing rock.

 To fully understand how this underground world was created, caverns offer scientists an opportunity to test their theories about the beginnings of life on Earth. There’s no moss or any life at all in here apart from bacteria. We have here in the ceiling uh this white sparkling thing which is a bacteria that only grows in in caves.

 So this bacteria actually grows because it’s been completely dark in here. It only needs uh water and and iron to to grow. So, it’s actually being studied, for example, by NASA. They study cave bacteria in Iceland. Away from the tourist trail, geologist Thor is stepping further into the cave than he’s ever been before to find the source from which the lava has flowed over thousands of years.

This is a huge chamber. This is the hole in the house. It’s at least 20 m wide and when it was active must have been about 20 m high as well. It’s a huge amount of lava that has gone through here. The length of it is on the order of at least 100 m. We see along here different stages of the lava flowing through here.

 So at some point it must have filled this chamber and then as the lava level dropped and we can see those different marks on the walls for me. This is Yeah, this is huge. So this is a narrow bit of the lava tunnel. Thor has reached a place that few people ever get to see. Other you see right there.

 What do we have here? Wow. Nearly 1,500 m from the entrance, the vent from which erupted the Earth’s molten core. This is absolutely spectacular. It’s a lava fall. The lava that actually created the lava tunnel came out of here. It’s just like a river system. Difference is it’s in the underworld. It’s actually in a cave. It’s enclosed.

We don’t see it at the surface. It all came from here. But as you go deeper in, it becomes more and more spectacular. Ruferholes Hadla is a stunning demonstration of primordial power. Superheated molten rock belching from the earth’s core, forcing through the crust and solidifying to leave behind caves of many colors.

 A magical sight to be enjoyed into eternity. You are in an underground world. This is a world in its own right. [Music] [Music] Deep inside a mountain is a staggering 55 km complex of tunnels. How did it play a crucial role for Allied forces in defeating the Nazis in World War II? 16,500 men deployed it deep under London.

 How will 21st century engineering innovation, this is something to be proud of, continue the legacy of Victorian underground pioneers? Thousands of people were dying in these outbreaks of cholera. And what unbelievable discoveries have been made in the depths of one of Europe’s largest and most stunning cave systems. It is extremely rare.

 This is the only place in the world where you can find them. Beneath our feet lie extraordinary spaces, caves and tunnels. The span and the size is just crazy. They’ve been designed and built by us. This is the only one with the castle as well as formed by nature. But how were they created and adapted? By who and why? You’ve got to face your fears.

 Throughout history, subterranean life has captured our imagination. Feel so privileged. We’re going further and deeper to unearth the mysteries, the stories, and the secrets of underground worlds. Along Spain’s southern shore, a limestone ridge dominates the coastline. Gibralta known as the rock is a fortified British overseas territory overlooking the straits between North Africa and Europe. It’s just 6.

8 km in area, but the size of its role in World War II is immeasurable. Beneath its distinctive landscape lie no less than 55 km of hidden tunnels and caves, a subterranean city carved out by a military workforce. 16,500 men securing this area against Nazi movement. How was this incredible feat achieved? Largest explosion brought it with 3,000 lb of jelly knife in one single blast.

 Why was the security of deep level bunkers so critical? In November 1942, the most important decisions were taken here in secret. And what World Wars have only recently been discovered. All the men were all sworn to secrecy. They were brought up here not knowing where they were. They would have bricked up the inside.

 Since first coming to Jibralta in 1988, former British soldier Pete Jackson has become an expert on its military and underground history. I arrived in Jibralta not knowing at that time what I was in for. Viewing it from the outside, it is almost impossible to fathom what is actually inside the rock. The Rock has been home to British military forces for centuries.

 It’s remained a British run garrison since 1704. It’s always been of key strategic importance to Britain sitting where it does in the mouth of the Mediterranean. You control the straits. You control what’s in and out. It’s a very important place. Since they arrived here, British troops have excavated through Gibralta’s limestone rock to join up existing caves and to excavate new tunnels and spaces.

Gibralta is a labyrinth of subterranean excavations which stretches such a time period. It stretches from the 1700s to post World War II. The system is fantastic. The majority of subterranean excavation on Jiabro was during World War II. At the beginning of the war in 1939, there were approximately 11 km of tunnels inside the rock.

 By the end of the war, troops had increased that to a staggering 55 km. It’s like coming to Mecca for anybody that’s an enthusiast for subterranean. We could almost call it a city underground. Janp’s father, Lawrence Bolton, was one of the soldiers who helped excavate this important subterranean city. She’s come to meet Pete at a place called Hayes Level to see the tunnels her father built for the very first time.

 The reason I brought us in here, we’re going to walk down through a series of areas that were drilled out to perform the duties of stores, which is exactly what your your father was doing when he was here, was producing areas to store. Now, a couple of years ago working in the south, I came across a sign laid on the side of the road, which I thought needed saving.

 And it’s another reason I brought you here, cuz I want you to see it. It’s where your dad was working. So, let’s take a walk down. We’ll show you that on the way. Be really interesting. We can see how high we are from sea level. The entrance to Haze Level is over 100 meters above sea level and home to the general stores.

 Rations, clothes, bedding, and equipment were all kept here. Everything needed for the 16,000 strong garrison to survive during the Second World War. A lot of this is what would have been used when your father was working here. One of Jan’s only momentos of her father is a photo of him using this type of hydraulic mining drill.

 You probably recognize this here from the photograph from your collection. The one that was drilling into the ceiling, right? That’s no lightweight piece of equipment, is it? It isn’t. It isn’t. I wanted you to see that sign. Pete’s research has shown that Jan’s father worked on an area known as Burma Stores. It’s an area that I have a personal tie into now cuz I know some of the history.

 It’s another piece of that jigsaw. Yeah. Yeah. Another piece of my jig jigsaw as well. It is their excavations which Jan’s father was part of were hugely important for the military purposes of Gibraltar. Jan’s come to Pete’s home to hear more about the role of the Royal Engineers. The Royal Engineers were actually formed here as a unit.

 What were the Royal Engineers doing here? Securing this this area against Nazi movement through into the Mediterranean was key to the Allied support. 16,500 men deployed here to defend, administer, and prepare this place for the duration of World War II. The work we were doing was was so important, Jan. I mean, um, had the rock fallen, then this route into the Mediterranean would have been lost to us.

The Royal Engineers blasted 43 km of carefully planned tunnels and underground spaces. As you go through the ages, the tunnels become far bigger for storage. Brigade headquarters for vehicle stores, generating stations, hospitals. You can enter the tunnels in the dockyard at sea level and make your way to the highest tunnels in Gibralta without putting your head above ground.

to travel through the rock undetected. The Royal Engineers excavated a gigantic main road from the north to the south. They named it after a familiar arterial route back in England, now known as the A1. And you know, on the Great North [Music] Road, they use things that people would recognize. You enter the great north road from its southern end at main avail which is where the A1 starts.

 We continue north within the tunnels as we would along the A1. We’ll get to Peterborough. We’re moving on from there within the tunnels. We get to Doncaster and then onto Durham. So these are towns that lay themselves along the line of the great north road in Britain. Anybody arriving from the UK entering this tunnel system, let’s say you had a young conscript, you needed to go to the stores at Peterbury.

knew that if he entered the tunnel system at Durham to get to Peterbury, he was going to have to turn [Music] south. This area is called Kalpyhole. It started like before World War II as a ration store. ration store was turned into Kalpyhole generating station and this was producing power for mod and RAF Gibraltar.

 What we have here inside are three Fuller engines. Engines like this can produce 1,035 kW per engine. Later on, a Rustin industrial gas turbine, a jet engine was brought in that produced a further 675 kW. This generating station is an unsupported chamber, creating huge voids in rock requires a phenomenal amount of explosive. The largest explosion known to have taken place on Jibralta with 3,000 lb of jelly in one single blast.

I was lucky enough to show a gentleman around who was in charge of drilling the largest chamber in Jibralta, what we call reie chambers. And he said that even his most experienced miners that were drilling it out said they didn’t know what the outcome would be. They said when they blasted it, it was either going to cut a chamber or it was going to cut the rock in half.

 Excavating large volumes of rock is highly dangerous and many British soldiers lost their lives supporting the war effort, including Jan’s father. We’re on our way into the rock to the spot where my father died. Lance Corporal Lawrence Bolton, an experienced miner, was killed while excavating a jiro tunnel when a detonation went wrong. This is the entry to Burma south.

 This is where your father would have been drilling where that tragic accident occurred. This is special that it’s quite it’s important for me to come and just identify with not the place where he died but the last place that he was alive. Pete can give Jan privileged access to the Warren of tunnels known only to the Royal Engineers.

 I’m trying to looking for graffiti in this wall. So, what you’re saying is then my dad’s my dad could actually have he may well have left his name in the wall somewhere in the wall. The area that we’re now standing in is called Upper Union Gallery, but it’s local nickname became Clappam Junction, right? Clappam Junction because just like Clappam in the UK, you can get almost anywhere from here.

 In tunneling terms, this goes north, south, east, west, left, right, up, down, in, out. In one direction from Clappam Junction, there’s access to a lookout post with the same view of the Spanish mainland Jan’s father probably saw over 75 years ago. Amazing. This is Jock’s balcony. Absolutely fantastic.

 That’s where my dad’s medals were presented to me by the Chief Royal engineer. Yeah. and all my family were there. It was one of the best days of my life. This is the first time I’ve been in this part of the tunnels and it just makes me feel closer to my father. [Music] During World War II, future US President General Dwight D.

 Eisenhower set up his headquarters in the depths of Gibraltar. Operation Torch was the first mass involvement of US troops in the European North African theater of war. From deep inside Jibralta, Eisenhower coordinated the successful invasion of French North Africa to reduce pressure on Allied forces in Europe and Egypt.

 During World War II, this must have looked very, very different. Luis Garcia works for a technology company that benefits from the security of this space once occupied by the famous US general. In November 1942, Dwight Eisenhower spent 12 days in one of the offices here implementing operation torch. It was very important because uh the most important decisions were taken here in secret.

 So a lot of intelligence gathering was done and the top brass was briefed of what the next move should have been. So a lot of very important decisions were taken in this room. Eisenhower’s command office was protected from enemy bombs by rock in every direction. We are literally in the middle of Jibralta, north, south, east, west.

 So we are in the heart of Jibralta. There was a tin roof in construction, so it must have been quite dismal at the time. According to Eisenhower’s autobiography, he wasn’t too impressed with the lack of fresh air and natural light. However, this space in the heart of the rock is perfect for its modern-day purpose as a data center. It keeps servers safe in a climate controlled environment.

 This was refurbished in the 1980s. Walking in obviously you cannot but notice that you are underground but once you’re in here it looks like a normal office. It’s just that we haven’t got any windows. During World War II, Gibralta’s subterranean world kept many things hidden. One of them, a top secret spy facility, remained undiscovered until 50 years after the war had finished.

Located on the southernmost tip of the rock behind rusting metal and debris, a hidden time capsule lay untouched for half a century. For years in Gibrala, there was rumors about this secret chamber inside the rock that was locally called Stay Behind Cave. People were often out looking for it. But in 1997, members of the Jibralta Caving Group were in this tunnel.

 They stopped just here to have a break and they felt wind coming through the corrugated iron. The tunnel’s enthusiasts had stumbled on their holy grail. So, it definitely showed they were in the right place. They moved further down and took another couple of panels away and they found the the way that the men would have been put into the chamber.

The explorers had uncovered a secret cave built to conceal British intelligence troops inside the rock should Germans capture Gibralta. The bold plan was to seal six naval personnel inside the hidden chamber. They would have bricked up the inside and people on the outside would have sealed the corrugated iron so there was no visible sign that anybody was inside.

The original plan was to hide the courageous men in here for 1 year. Provisions and equipment were supplied to last a whole 7 years. Tracer Cave was built to last with two toilets and two ventilation systems operated by hand or a kind of exercise bike. The men could charge batteries and stay active while remaining completely silent.

 It was a static bicycle and instead of having a chain, it had got a leather band so that it wouldn’t make a noise. The men would need to stay in good physical condition. To keep the noise down on the floor, there are cork tiles. To den the sound, by far the most exposed part of Jibralta’s Tracer Cave were the lookout posts.

 One pointing east to the Mediterranean and the other west out to the bay and the Atlantic Ocean beyond. The original plan for the apertures was for them to be 30 cm wide and 15 cm high. When they got to the east side, this is part natural cave. There’s a small natural platform on the outside and the men could actually climb outside.

 They’re shielded from view from the Germans anywhere because there was a lot of foliage around it. So they could actually get outside, get some sunlight and monitor the shipping from there. But on the western side, it was very exposed, very close to where there was a path that the Germans would be patrolling. The slot itself is only about 15 cm long and 2 cm high.

 They had a concrete wedge, which normally would be slid in there with a tiny pin prick hole. They’d look through there, check that there were no Germans in the area, and then they’d carefully remove the wedge. Once they’d gather the information, they would need to transmit it back to London. But that would come at great risk.

 They couldn’t have an aerial sticking out of the side of the rock because it would have given away the position. So it’s a flexible aerial and during the day it would have been pulled into that pipe there. And then at night they would compress the information down so you could send a lot of information in a very short time using a specific code.

 They would send a very quick transmission, receive any orders, and then shut off. They wouldn’t want to transmit for a long time. Luckily for Jiroto and the Allied forces, Tracer Cave was never needed. 300 m above sea level and part of the Upper Rocks nature reserve, there’s another stunning space, St.

 Michael’s Cave is one of Gibraltar’s greatest natural wonders. During World War II, the cave was prepared as an emergency hospital, but never used. Today, it is a unique auditorium and for some, like this 11-year-old choir member, a brand new magical experience. Actually, I’ve never performed in St. Michael’s Cave before, so this is my first time.

Kind of looks like, you know, ice crystal palaces. It kind of looks like one of those, but with rocks. Sus caves is one of those magical venues that you take for granted when you live here. This place is is really something special and very unique. The acoustics are amazing. [Music] I sounds really really cool.

 This is part of our history. So we get to sing in a place that has so much background. This is special for me to come. I’ve realized that it wasn’t in vain. I didn’t know anything about him and I found him here. [Applause] They have this respect for the space. And thanks to you, I’ll speak. I’ll rise above. I am enough.

[Applause] Slovenia, a central European country situated between the Alps and the Mediterranean Sea. It’s 20,000 km in size, which is 20 times smaller than California, but beneath its surface are a staggering 11,000 caves. Until relatively recently, they were hidden for millions of years. I found paradise, the biggest discovery in Slovenia, if not worldwide.

What mythical creature was found thriving in this subterranean labyrinth? The first people that found it, they actually thought it was baby dragons. And what else lies hidden beneath? It’s important to explore, to discover new unknown territories. It’s pushing us forward to find places where nobody else has been.

Poston’s caves were discovered in 1818 when a local man called Lucetch was given a job in preparation for a visit from the emperor of Austria. The task of Luca was simply to climb as high as possible to put a welcome sign to the emperor. As he walked up the wall, he discovered a narrow passage and the rest of the cave.

 He said, “Uh, I found [Music] paradise.” It was back then the biggest discovery in Slovenia, if not worldwide. Lucachet had discovered what is to this day one of the largest cave systems in Europe. At 24 km long, it has been created by the Puca River, which eroded down through the limestone rock to create this stunning complex of caverns, halls, and passages.

Pine Cayama was actually formed in a lot of limestone layers approximately 80 to 90 million years ago. The biggest cavern carved out by the Puca River is known as the great mountain. [Music] The room itself is 100 m long, 40 m wide, and more than 30 m high. Inside this big room, you have really big cave formations.

 Steltes and stallagmites are formed by rain water dripping from the ceiling. This water will be able to dissolve and transport the limestone to the inside of the cave and the limestone will solidify into various shapes. And the one that you can see above us, we called it Nebotichnik, the skyscraper, 16 m high, the highest one in Pinama.

[Music] These incredibly precious formations have been here since long before man ever walked this earth. Some limestone layers are 90 million years old. The average speed for the growth of cave formations in Pine Cyama is approximately 10 years for 1 millm. That is 100 years approximately for 1 cm.

 The oldest dated cave formation in Pineska. It’s around 540,000 years old. Postonia is important to geologists from all over the world who study speliology. The science of exploring and studying caves. The geology in limestone and post skyama is perfect because water will carved and formed uh opening space and limestone is solid enough not to collapse.

 Lucach had discovered something that not only the Austrian emperor was impressed with. The complex quickly became a hot spot for tourism. In 1872, a railway with 2,260 m of track was laid from the entrance to the great mountain. It was one way only and it was sort of a push trolley for four persons that was pushed by a guide.

 The popularity of Pastonia’s caves brought further excavation and investment. By 1923, the train was motorized. This was the world’s first cave train and the system even had its own post office. The magic of these caves drew people from across the globe. For already 200 years, people are coming from all over the world.

 You know, from China, from states, from Greenland, Iceland, from really everywhere. One of the biggest attractions was and still is a stunning limestone structure that stands an impressive 5 m tall. It is simply called brilliant. The name of the formation is the brilliant because it’s so pure white is made of limestone.

 Pure limestone, no impurities. The discovery of Pestonia’s caves was hugely significant, not just for tourism in Slovenia, but also in the fight against one of the world’s deadliest diseases. In the deepest, darkest parts of this cave system lives a mysterious creature. The first people that found it, they actually thought it was baby dragons.

So this is the biggest cave animal in the world. They’re actually top of the food chain. They can live up to a hundred years and have completely adapted to living in the cave system. They have very good sense of hearing and of smell. So that’s easier to find their prey. And they prey on all the other cave animals that we can find here.

 They don’t have eyes. They hunt by detecting the bioelectric fields of other organisms. Here you can see the head. If you look closely, you’re going to see the heart which is basically located in the throat. Behind its head, they look a bit like uh red ears. These are the external gills, the whole darker part of the body. These are the intestines.

 If you take a closer look at the front legs, you’re going to see it has only three fingers. On the hind legs, it only has two fingers. These creatures can live for up to 12 years without feeding. So, it is extremely rare to see cave salamanders actually eating in nature. So, you have to consider yourself lucky to get such a good shot.

The more these creatures are studied, the more they reveal about themselves. Something very special happened in this aquarium. The mama actually started to lay eggs. Yeah, she laid 64 eggs. 21 of the young lings hatched. And this is actually an event. It has never before been seen in nature.

 We were very lucky that it happened in this aquarium. These cave salamanders also had a genetic surprise for researchers. They may be immune to cancerous cells, so they can’t get cancer. And this is the only place in the world where you can find them. So there’s still much research to be done on this subject.

 I mean, it’s amazing to comprehend that in such an environment that anything would actually [Music] survive. The search for secrets hidden in the depths of Ponia’s caves continues. Starting from the limits of recently discovered networks, cave divers like Igor Rahovich search for what else is waiting to be discovered.

 Today we are going to do exploration of fifth samp of this cave. For Postonia’s cave divers, each new expedition is another step into uncharted territories. Caves is the only place in the earth now where nobody has been. We don’t have satellites to film the underwater caves. Diving into the unknown is timeconuming and tiring. Complete exploration.

 It’s going to last about 11 hours. But this underwater part is going to be like hour and a half. It’s highly dangerous. It’s like hostile environment. You’re underwater. It’s dark. But it’s also rewarding. They regularly see the cave salamander in its natural habitat. It’s magical.

 For the people who spend their lives here, there’s a real sense of purpose to push further and deeper. There is so much more to discover. [Music] [Music] Across London, an ambitious engineering project is taking place. Population of London was 2 million. We’re now at close to 10 million. Giant holes are being bored down deep under the capital.

 When they’re up and running, the guys can excavate 1 meter in an hour. They’re connecting to a vast historic underground system rarely seen by people above. This is a massive piece of pumping work. This subterranean work of genius has saved millions of lives. What Joseph Basiljet did was save a city. You can actually see individual chisel mark.

 How was this system constructed with Victorian technology? Having an integrated sewer system, I mean this was radical. And how is it being futurep proofed for the 22nd century and beyond? That 40 million tons of sewage that goes into the river every year, that’s going to stop. 29 vertical shafts like this one in Hammersmith, West London, have been excavated to be linked up to an epic brand new 30 km underground urban wastewater system. This 4.

2 2 billion pound subterranean network will run directly beneath the river temps serving 13 burers and the city of London. Sewer expert Colin Fen is meeting engineer John Kakorin of the temp’s tideway project. [Music] John, I’ve come down this huge shaft which it’s so many steps deep. How far below ground are we? Well, we’re 35 m deep and it’s a 12 1/2 m diameter shaft.

How on earth did you construct such a shaft like this? So, we dig out a meter. Then we spray a line in. We spray concrete. Then we continue do another meter. Spray spray hit that lining and carry on down. The main tunnel is 300 m that way. or will be ultimately we’ve completed this connection tunnel. So this is 300 m long and later on this year underneath the river temps the larger main tunnel with the tunnel boring machine will come through eat off the side of our tunnel just the very end of it.

 Whilst the larger main tunnel boring machine spans over 10 m the smaller feeder tunnels need their own agile tunneling equipment. This is the specialist excavator we use. Call it a sha excavator. It runs on electric. And the two big features of this which make it good for tunneling. Uh one is the special knuckle joint on the arm here which means you can dig that tunnel profile.

 And the second one is all the spoil comes into this conveyor system. So the excavator driver doesn’t have to keep rotating. The guys can excavate one meter in an hour. We actually have four or five dumper trucks. This machine keeps them busy basically. They keep loading it and they take it away as quick as possible and it’s taken away by surface muck wagons and used for land reclamation.

Now, this tunnel has been designed to last 120 years, and we want to absolutely minimize the amount of maintenance because it’s very, very difficult to go into a sewage system once it’s operational. Currently in London, when there’s heavy rainfall and the storm drain network fills with water, it floods the sewer system, meaning raw sewage escapes into the temps.

 The pumping station here is activated on average once a week. So over the course of one year, that’s 2 million tons of sewage going into the river, which we will intercept. So any fish in the river, anyone canoeing, anyone dipping their hand in the temps is likely to have a bit of a surprise 50 days of the year. I’ve talked to people locally around Hammersmith and they know when the sewage is coming out because it it kills the fish.

 We’re going to do something about this and have this fantastic project within a few years time we’ll capture all that sewage and we’ll have a much better cleaner river temps. This is a massive forwardthinking underground engineering project for the public and environmental good. It builds on the groundbreaking work of visionary engineer Joseph Basiljette who created London’s existing sewer system back in the 1860s.

 Well, if you look back when Basilgette designed his 150 years ago, the population of London was 2 million. We’re now at close to 10 million. One reason for Temp’s Tideways construction is the rapid expansion of London over the last century. Joseph Basiljette sewers were built because London was growing, but more importantly because people were dying.

Sir Peter Basiljette is Joseph’s great greatgrandson. It was in about 1831 that the bacterium cholera arrived from India. Lots of people drawing the drinking water from the tempames but their sewage was flowing out of their houses into the temps via the tributaries. They were drinking their own excrement.

 Thousands of people were dying in these outbreaks of cholera. It actually would have got worse and worse and worse had there not been a cleansing of the city and a citywide solution. This is Cross Nest sewage pumping station on the south side of the river Tempames away from the city. It’s where Basiljette’s mammoth subterranean system culminates.

 This is a cathedral, a cathedral to municipal engineering. We’re at the end of Basiljet’s major sewage improvement works that he’d started in 1856 across London. So here we have a map of London showing the Basiljette sewer network all in place. So firstly in the north of London there are three main sewers. Uh they all meet at Abby Mills pumping station at the top.

 Then in South London we have three more and they come flowing down takes the waste and puts it out across Ness where it then hits the more tidal reaches of the temps and it can hold the waste and then release it as the tide goes out. Cross Ness was the southern guardian of that sewer network. It was a matter of huge pride and at the opening we saw Prince Albert and lords and ladies came here.

 So it was something to boast about. It wasn’t going to just be a anonymous building hidden away in the marshes. We’re surrounded by the cast iron tracery. And then all around here we’ve got beam engines. beam engines with pumps that are being used to move the effluent that’s been taken through the sewers. The pumps were used to lift effluent from the sewers deep underground.

 It was held in pens and then 75,000 tons of sewage would be released with the estie tide out towards the North Sea. This is a massive piece of pumping work. Everything here is just solid. It’s just gorgeous. This is something to be proud of. Peter. Hello. Hello. So Colin, here’s our cathedral of sewage. Basel was hardly talked about, hardly known compared to say Brunell because most of his achievements are underground. You can’t see them.

 There was an article in one of the newspapers, I think it was the observer that called him the drain brain. And then having read that I realized quite how profound his achievements were. London at the time had a population of between 2 and 2 and a half million. And when Basiljette finally got the money to execute his the scheme um he overspecs the whole scheme for a population about 4.5 million.

 It allowed London to grow as a sort of economic entity. What Joseph Basiljette did was save a city. He was a true visionary and Joseph Basiljette enabled a healthy London to thrive. But his underground engineering genius wasn’t a first. Decades before in the 1830s, 200 m north of London in the city of Liverpool, a civil engineer called James Nuland was about to create the world’s first integrated subterranean sewer system.

 If we were to think about Liverpool in the 1830s, it was filthy. You had butchers slaughtering animals in the street, fish mongers throwing the guts around and hoping for a heavy rain to maybe wash them down into the Murzy. There was a sanitary report and it highlighted Liverpool as being particularly filthy. The medical officer noted that the poor were living in dreadful conditions in basement.

 In fact, he said, “I found them inundated with fluid filth which had ooed through the walls from adjoining ash pits and ces pits.” As an important port for the industrial heartlands of the north of England, Liverpool’s reputation for scholar and disease started to affect its trade as a port. In 1847, James Nuland was appointed the first burough engineer.

 From this it went from the basist form of filth and disgusting environment to really the heights of modern sanitary thinking. James Duland turned out to be in some ways the sanitary savior of Liverpool, didn’t he? Yes, he he definitely did. When he arrived in Liverpool and was appointed, he targeted a lot of specific issues such as street lightning, pavements, communal bathous, which all played the small part of ultimately cleaning up Liverpool in conjunction with a major sewage system, which he created, the first of its kind. Construction began in

1848, and by 1851, 27 km of main sewers had been built. This increased to a staggering 480 km by 1869. The construction of the world’s first integrated sewer network completely changes the health and the outlook for Liverpool. The the fact I think the average age before systems like this were built life expectancy in Liverpool was 19.

 However, just by building these and putting these in place, that over the 10-year period it took to construct doubled. The life expectancy doubled. Liverpool’s pioneering subterranean sewer project had transformed the lives of its people, and it wasn’t going unnoticed. This was a model that was then going to be picked up by other towns and cities, most notably across London to manage this new modern sanitary revolution. It was designed to last.

 It was designed to allow the city to grow. This was a standard design in the Victorian era and it was built using an open cut method. So this would have been exposed built by the various trades and laborers and then eventually once it’s complete then they backfill the ground above it. You can actually see individual chisel marks where each individual stone was handcrafted.

 We’ve got bricks on the bottom and then a a stone arching over the top. Yes. And then that comes onto a brick channel on the bottom that’s carrying the effluent around. Yes. keeps the solids in the middle where the velocity is the greater. The actual gradient is one in 113. So that actually gives it the potential to selfcleans.

 So the flows will be enough to ensure that it’s kept as clean as you can see. At peak flows, this sewer can handle 32 million L per hour. Enough to fill an Olympic size swimming pool in 3 and 1/2 minutes. the age that it is at 150 plus years and the condition and the cleanliness of it suggests he might have had a good idea.

This is the first time anyone had contemplated having an integrated sewer system across anywhere in the world. I mean this was radical. It was a system that many other people followed after and ultimately it was built for a reason. It achieved that reason and it’s still standing. So it is quite magnificent. The remarkable thing about Liverpool sewers is that we can still see parts of that original Victorian brick network integrated in to modern concrete systems.

 The Victorians created revolutionary underground networks that made Britain healthier and allowed it to grow. We need today to understand as much as they did in the Victorian era that great public works are a legacy. This is something that people couldn’t have imagined years ago. The technologies, the way that it’s all come together.

 And I’m looking forward to that day when we start seeing wildlife coming into the river, you know, and who knows, we could see dolphins coming up and down the temps. And won’t that be fantastic? This infrastructure has been designed to last not just our generation, not just our children’s generations, but for generations beyond that. [Music] [Music] [Music] Beneath a capital city, giant cold war atomic bunkers.

 Nuclear war can come tomorrow. How has one of them been transformed? This is the ultimate underground world. Deep within limestone bedrock, cavers continue the excavations of a pioneering subterranean entrepreneur. got this hidden network and new treasures to find. And hidden beneath the surface, this is one of our secrets. Inside a 1.

2 km thick layer of salt sits a world first. It’s amazing here. Beneath our feet lie extraordinary spaces, caves, and tunnels. The span and the size is just crazy. They’ve been designed and built by us. This is the only one with the castle as well as formed by nature. But how were they created and adapted? By who and why? You’ve got to face your fears.

 Throughout history, subterranean life has captured our imagination. Feel so privileged. We’re going further and deeper to unearth the mysteries, the stories, and the secrets of underground worlds. Sweden. It’s the largest country in Northern Europe, sitting strategically between Russia and the West. The Swedes are a peaceloving people and fiercely independent.

 This is a nation that has taken extraordinary precautions to protect itself. We had to prepare ourselves for a war. By excavating an astounding 65,000 nuclear bunkers, everyone in in Sweden knew that the war could come. It would have been quite terrifying. But how were these impenetrable cold war subterranean spaces created? They had to blast 65,000 cubic meters of bedrock.

And how are they used to protect Sweden today? These machines take over if the grid fails. It couldn’t be any better. In 2007, underground explosions shuttered through central Stockholm. 40,000 cubic meters of solid rock were being removed to tap into and extend a vast underground network that was originally dug out in the 1940s.

 I found this facility and it was like yes. John Carong was creating a unique workplace for his company. I remember I was standing in a pile of rocky rocks everywhere and I thought oh no what have we done it it’s it was an extremely big project to achieve and this is also in the middle of the cities we have to remove the rock uh it was extremely hard to do this John’s business stores and protects Sweden’s digital information in what are known as data centers whatever you do on the internet is located in a data center. So when you

use your cellar or you use your computer and you go out on the so-called cloud somewhere, it’s always located in the data center and that needs security. He’d found an underground location where security was built in a subterranean space that used to be a cold war nuclear bunker.

 Historian Pier Olgen is meeting John at the Pionan facility. Hi. Hi. Welcome. Thank you. to explore his vision of future security. Oh, this is interesting. Yeah, this is amazing. Why Why do you have blast doors? What do you What do you need them for? It’s a matter of security. This is ultra high security facility.

 So, this is the reason that we use them. I like that you kept raw rock walls. It also shows you a bit of how this place used to be when it was used as a shelter. Actually, we have an alarm as well. We have a a second world war German sub alarm here as well. Alarm alarm alarm. And then then this this sounds I like the sound of it.

 Sweden was preparing itself for a war that never came. If people wants to learn about the second half of the 20th century, they just have to open a door to one of these shelters. I wanted to keep something that you could get the feeling of the Cold War security, the bunker feeling, but take it to the next level. What did it take to convert this relic of the nuclear arms race? Architect Albert France Lenor was tasked with making John’s vision a reality.

 The brief was do something spectacular and something sci-fi related. But honestly, The Rock does most of the job itself. I think this is a very geekish word. People who work with the technicians, the people who work with the stuff, they’re very geekish. And as me, we love science fiction. We love these kind of movies and I think this this the design ideas come from that.

The data center now boasts waterfalls, a vertical garden, low-lying fog, and a climate controlled suspended glass conference room. In the the most spectacular room, we realized we really need to make this space accessible uh for visitors, for clients. In the server room you have a specific climate and because of that you need to cool down the server and that makes a lot of noise and that’s how the idea of the hanging glass room start to grow because I was like okay the room with its own climate within the server

room. Visitors to the pian facility are often struck by its resemblance to the bad guy’s layer in an old James Bond film. People have always asked me if I want to buy a white cat and sit there sit there in the middle like some kind of movie villain. This re-imagined subterranean structure still plays an important role in protecting Sweden.

 Today it’s not only people that is important to protect. It’s also the machines that people uses. You have your physical everyday life, but you also have a kind of a digital life. And this facility is all about protecting the digital life. The facility is designed so that the power supply never fails.

 Two giant backup engines stand ready to produce 1.5 megawatt of power in the event of a crisis. These machines are auxiliary power engines that take over if the grid fails. and provide this facility with energy. It’s V16. There are about 27 L and 1,200 horsepower. When the machines starts, it’s awesome. There is no exception. It’s absolutely the coolest facility in the world.

 So, this is the ultimate underground world. It couldn’t be any better. These places are part of our history because they can explain to us in a very evocative way the world of the Cold War. What it was like to live during the Cold War era and know that the nuclear war can come tomorrow or the day after that. During the Cold War era, from the mid 1940s to the early 1990s, Sweden wanted to protect itself from attack.

 The strategic position of Sweden, we had to defend our territory and defend the civil structures of society. Many countries in Europe, among them Sweden, started to shield the population so that people could survive an attack. Here in the center of Stockholm underneath a church is Claraara Shelter. [Music] It has a sign that every Swedish resident knows well.

 This symbol uh orange square and the blue triangle is you you can see it all around Sweden in houses and along streets and it tells you that this is a public shelter. Claraara shelter is a concrete structure that is encased and protected by the granite bedrock of Stockholm. We are standing in the main shelter tunnel and this is actually a house within a cave and the point is that you have the ability for the rock and the building to move separately.

 So if there is an atomic bomb blast above our heads, the the rock will shake but the building will not shake with it. Built between 1958 and 1961, it can be turned into an emergency population shelter in less than 48 hours. It’s one of four large population shelters. And this place could house up to 16,000 people. Its architect, Alan Vanna, designed a long spiral tunnel that splits into two levels.

To make Clara shelter, they had to blast 65,000 cubic m of bedrock. To build the shelter itself, they needed 12,000 cub m of uh reinforced concrete. 15 m below the surface, the shelter would be accessed from the streets and subway above. If the air raid sirens went off, people would rush down the many stairways from streets from the subway and then come into the lodge tunnels and then the blast doors would close.

 One can only imagine 20,000 people down here and uh the light starts to flicker and the ground and the walls shake. It would have been quite terrifying, I guess. If the shelter had ever been needed, civilians would have found themselves guided safely into the refuge. People in the center of Stockholm was directed down these stairways by civil defense people.

 The color coding on the railing here is used so that the civil defense people can show different kinds of people the right way down. You have to imagine hundreds even thousands of people rushing down these stairways. You’re a pregnant woman railing. Uh old age pensioner green city official red. It has to be quick and the sirens are sounding up in the street and then they can avoid panic and make everything run as smooth as possible.

 Doors made of steel and concrete weighing 70 tons were built to withstand a nuclear blast 1 kilometer up in the atmosphere. This is the main blast doors for the shelter. And uh in case of an emergency, they would shut automatically. Uh and they are powered by small diesel engines. And when they are closed, the plates on the ground flips up and the doors closed.

 And that is how you seal the place off because the blast from a nuclear explosion consists of course of of wind and everything but the pressure the air pressure rises very very quickly. So the doors would have to handle that. These specific doors they can close in two or 3 minutes. And it’s made to protect people from the blast, from the radiation, and from poison gases, uh, fire, and so on.

 In the event of a nuclear attack, Sweden’s citizens were advised to be prepared to survive down here for 1 month. People were supposed to bring with them blankets and some food, canned foods, bread, uh, and so on. Everything down here is supposed to be functioning even after the municipal power has been shut down. Wells, water systems, air purification facilities, reserve power plants.

 This place is supposed to be totally self-sufficient. The survivors couldn’t have stayed underground forever. You would eventually have to send someone up to the surface to check for radiation. So that person would then dress in a hazard suit and venture out with a geer counter and measure how danger it is.

 And then when they come down, they will not be allowed in to this sealed space. They would have to speak via these kind of systems to the guards inside. And these are for when it’s okay to come inside and when it’s not okay to come inside, green and red light. And if everything is okay, the inner doors are open. So there was a system for gaining entrance to this place.

 Once the large blast doors were closed, Claraara shelter is now used every day as an underground garage, but Sweden remains alert to [Music] threat. Four times a year, it tests its public warning system. In 2018, Sweden’s government issued emergency guidance for its people called If Crisis or War Comes. It was sent to 4.8 million households.

 Sweden remains prepared. In southwest England, deep in the Menddeep Hills of Somerset, lies one of the most spectacular landscapes in the [Music] UK. Cheddar Gorge was formed more than 1 million years ago when melting glacial waters carved into limestone rock. It left behind near vertical cliffs and a mysterious subterranean world.

Beneath this breathtaking landscape lie secret caverns and magical rock formations. Can you imagine seeing this for the first time? You’d be like, “Wow, what’s this?” But how were they created? Why did humans come to explore here? Richard Gosoff was looking for a show cave. And boy, did he find one. And what can we learn from its hidden secrets? These tight squeezes make visibility quite difficult.

 You got this entire hidden network and new kind of treasures to find which makes the whole lot worth it in the end. The gorge is almost 120 m deep and 5 km long. At its heart is one of Britain’s oldest underground worlds. Cheddar Caves. This is one of the best caves in the country. So the whole cave is a friatic cast system which basically means it was formed by one big river which is now sunk below this cave system.

 All of this formed by the pressure of water. It’s amazing to see what water really can do. [Music] The story of this magnificent cave system begins at its entrance with an extraordinary discovery of a 10,000-year-old complete skeleton of prehistoric man. A search to find the closest ancestor of the skeleton known affectionately as Cheddar man sparked the interest of local teacher Adrien Target.

What I really wasn’t prepared for was the extent to which this story suddenly became global. DNA extracted from Cheddamman’s tooth was tested against local students from Adrian’s school. But the test threw up an unexpected match. We were told there was only one result and the man who was interviewing me said, “And it’s you.

” And he looked at me and said, “You’re the one.” So it was a bit of a shock. He was found very close to the entrance. So it’ll be interesting to see how we can compare what the caves were like then with what we see today. Paul Hemington is an expert on these caves. He’s meeting Adrian in the very place where Cheddman, his ancestor, was found in 1903.

Well, he was a hunter gatherer. So they would have gone out to hunt for their food, horse and deer at that time and would have gathered fruit, nuts, seeds, berries, and as the sea wasn’t too far away, they would have had fish on the diet. Although fishbones are hard to find evidence of, but they would have certainly had limpetss, molllesks.

 So that would have been quite a good diet actually. Very good, actually. Yes. Yes, I’m kind of on it now. Yes. It’s very very good because of course there’s no refined sugar and deer is very healthy. It is indeed. Yeah. Very nice too. In the winter time of course things would have frozen over.

 So it is believed really that they may well have been outside of the cave would have been in a kind of yurt type tent and then during the summer they would have resided in here. Uh this is the the river now that formed this cave has now dropped. It runs underneath and the whole reason why Cheddar man was discovered was because this was to alleviate the flooding that obviously still happens to this day.

Cheddar man was discovered after a new wave of tourism during the reign of Queen Victoria. The middle classes had more time and money on their hands than ever before. The Great Western Railway arrived here in Cheddar in 1869. So people in their finery, they they would come here and walk up and down. Local man Richard Goff saw an opportunity to take advantage of the large number of tourists flocking to Cheddar by opening the caves to the public.

 Was a really really a bit of a rack on very enthusiastic was looking for a show cave and boy did he find one. Though the caves were known about before Gooff’s time, there had been no opportunity to explore the full extent of this magical world before he dug it all out. With the help of his two eldest sons, William and Arthur, what’s unique about this underground world is that the limestone is marine limestone.

 It was once on the seabed when the UK was under a warm tropical sea near the equator. So it’s crushed dead organic matter which contains krinoids, coral and brachopods and then thrustful and fold. The mendits were formed and here we are now. Richard and his sons have broken through this massive boulder choke blasted through about 8 yards to access and open out the rest of the cave.

For 2 years, they continued this painstaking work on their hands and knees into the unknown. His first discovery was like a gift from the gods. Richard Goff called them the fonts. Can you imagine seeing this for the first time? You’d be like, “Wow, what’s this?” Formed over thousands of years, pure white crystals called calite are deposited when rainwater with dissolved limestone evaporates.

takes about a thousand years for a cubic cm to form. Cowsite is white in its purest form and the red is iron oxide which is all mixed together to give that wonderful sort of dry waterfall effect. These nine basins range from a few inches to 4 ft and are called gora pools. They name the fonts after what you would see within a church.

 the fonts that I used to baptize babies with. That’s what they reminded him of. This is the power of nature. This is what nature can do. This is what water can do. And he’s probably thinking, where does that lead to? It it makes you feel quite quite small in a way to see the wonderments of how this has been formed. Goff continued to come face to face with mother nature’s beauty.

Right. I absolutely love this. This is obviously like a copper beaten effect. Uh the actual name is scalloping. So this is where the river has reached the ceiling of the rock. It’s not allowing it to take any more out in certain areas. So it’s lapping against the rock and creates this wonderful, as I say, it’s like a copper.

 This is entirely natural. Yes. Yeah. Absolutely. Cuz it’s it looks beautifully delicate. It does. It if a craftsman had beaten it almost, doesn’t it? Exactly. Right. It really is incredible. On his hands and knees, Richard Goff continued excavating tons of rock and silt on his mission to riches.

 100 m from the font, he arrived at his next marvel. Oh, wow. That’s magnificent, isn’t it? Isn’t it just just imagine coming through calling four years to get to this point on your hands and knees and you come into this. [Music] Goff called his next discovery St. Paul’s after the famous cathedral in the city of London. Absolutely.

 It’s a fantastic spectacle, isn’t it? It It really is. It just opens out. So, you know, it looks liquid in places. It does. That’s right. Well, this is an amazing view. One can’t imagine really how inspiring it must have been to have seen this natural phenomenon complete with uh its delicate shapes but also the different colors as well.

 It must have been really all inspiring to have seen that for the first time. Goff’s desire to open the caves as a spectacle for tourists was realized once he broke through to the diamond [Music] chamber. By 1899, splenders that were once lit by candles and gas lamps were now brilliantly displayed in electric light. Okay, Adrian, check this out.

Wow, look at that. That’s amazing, isn’t it? Absolutely. If I had a pound for every time somebody got to that point over there and said, “Wow, I’ll be a rich man.” The diamond chamber is 28 m high and is thought to contain 60,000 tons of limestone. It’s the last and largest of Goff’s discoveries and home to an eye-catching formation.

 Niagara Falls formed when a stallctite and stallagmite joined [Music] together. It’s believed that the cave dried out 250,000 years ago. It’s a quarter of a mile in. So, it’s like a journey. That’s was a journey through time. Yes. And nature itself and that’s really what makes this so so special. So Richard Goff to reach this point it took him 8 years of his life and he was 71 by the time he reached this end chamber.

While prehistoric humans only ever ventured to the mouth of the cave, Goth and his sons uncovered majestic spaces once hidden deep within. Still to this day, explorers continue to search these caves just like Goff did in the hope they will find the next diamond [Music] chamber. One of these cavers is Sam Rasmuson.

 It’s not really for the faint-hearted. You’ve got to kind of face your fears as you go through the cave. To see just how challenging Goff would have found navigating the Cheddar cave systems, Sam has offered to film himself on a small camera. Uncharted caves can be very, very tight. They could have loose rocks as you go through.

 So, of course, you do have the risk of rock falls perhaps happening. There are certain passages you could go down and they suddenly drop out say 40, 50 ft. So, you need to be prepared for these things to happen. Um, but of course we can only be so prepared. Going further in, of course, there will be no natural light.

 Wave your hand in front of your face, you won’t be able to see it at all. You can open and close your eyes, it’ll be exactly the same. You won’t see any difference at all. Sam is entering the black cat chamber. It’s a part of Gooff’s cave not open to the general public unless with a guide. get all these different colors from the different minerals coming down through the calsite.

 So, it’s mostly brownie red from the iron oxide or red ochre making its way down. Hidden from view, the further wonders of the cave are reserved for the bravest of visitors. Crawling through these narrow spaces can be both uncomfortable and potentially dangerous. These tight squeezes make visibility quite difficult.

 You see small gaps that you perhaps want to give give a go at trying to get through. It’s not really for the faint-hearted. Um, you do have to have that sense of adventure. You’ve got to kind of face your fears as you go through the cave. And these are the places you look for when you go caving. Finding something like this is a huge reward for your physical efforts.

and coming out on the other side seeing something that perhaps somebody has never seen before. It’s it’s quite kind of sensory uh depriving really and it’s quite damp in there, but with the lack of light, you hear certain sounds a little bit more. So, you hear strange sounds as you’re going through the cave.

 See some evidence of excavation here and there as well. Nice little passage. Experienced cavers are still exploring new areas of the system to this day. There still are passages still being discovered. We’ve still got dig sites going on through the gorge and even in this cave as well. Got this entire hidden network of um kind of tunnels and and new kind of treasures to find through these cave systems which makes the whole lot worth it in the end.

These magnificent caves continue to attract thousands of people from all over the world and adventurers who continue to explore this stunning complex of caves. It’s amazing to think that Chedderman who resided at the entrance to the cave never realized that he was on the threshold if you like of a fabulous underground world.

[Music] Romania, southeastern Europe, bordering the Black Sea. A land mass of 240,000 km ringed by the great Carpathian mountain range. But beneath its landscape lies one of history’s greatest riches. This location is actually one of our secrets. How were huge caverns created? They needed almost 200 years to create this huge hollow shape.

And after centuries of mining, what have they become? You have no idea when you’re above that underneath your feet, you have an underground world like this. 13 million years ago in what is now Romania, a prehistoric sea ringed by mountains evaporated. It left huge salt deposits covering approximately 45 km.

The history of this place is pretty amazing. The Romans settled here in the ancient city of Potesa. Historian Paul Florenurion has been studying the excavations of salt here that date back to the time when Rome ruled Deisha, now modern Romania. They discovered the salt resources of the area and they started to mining up.

Salt was prized for its ability to preserve and season food. In fact, we get the English word salary from the Latin word for salt. They will dig a hole and after a couple of years, the hole will fill up with water and they will just continue digging another and another and another. These early digs barely scratched the surface of the enormous resources later discovered deep underground.

 Some areas of the deposits across Romania go as deep as 1.2 km. Mines expert Edmund Christian Pa explains the techniques used by medieval pioneers to extract salt. This is the marks left by the tools. So they were using hand tools, chisels and hammers. It was something like this. They were considered quite efficient when they were trying to cut blocks of equal size.

With simple tools, workers excavated the salt by digging ever deeper. The deeper they dug, it became more difficult to lift the heavy salt out of the mine. So an ingenious device was created to do this. This piece of equipment we see here would have been powered with the help of horses. They were attaching one or two horses been able to slowly turn it.

 Up to 500 kg of salt could be lifted out of the mine in one go. They are connecting this mechanism with a pulley system. The idea of using uh a system like this one was first applied in 11th century. It was an approach used for almost 800 years. It is essentially the grandfather of the modern elevator.

 So this is uh the shaft from where they were extracting that loaded sack of salt. Underneath the bullet system there is a shaft which is very very deep about 90 m deep. Large quantities of salt would have been brought up to this platform loaded into trolley and these trolley were pulled by horses through the tunnel directly to the surface.

 So we can see here the railway slippers. They were using a narrow gauge railway system for transporting salt to the surface with the help of special trolley. The tunnel is perfectly straight in order to make it as effortless as possible for the horses to transport salt to the surface as the load itself was already very very large. The technique of creating deep bell-shaped mines below a central winch was used at several locations over the centuries.

The steep-sided walls of these mines meant they wouldn’t collapse as workers excavated ever deeper. Paul Floren has joined Edmund to see the deepest of them at Selena Tura, the Tourettia mine. This is one of the bell-shaped chambers. It’s one of the deepest and the oldest one from around here. Mining began here in 1690 and took almost 200 years to create this huge hollow shape.

 They used only uh simple technology to mine salt. That makes it more amazing, more impressive. [Music] The base of the mine is 112 m below the surface and 75 m across. Due to the infiltration of water, an 8 m deep underground lake was formed with a salt island in the middle. This island is an artificial one. It was created in the middle and the end of the 19th century.

Blocks of salt that failed to reach the standards they were looking for were thrown back into the Territzia mine. Over the years, it’s formed this impressive island that now stands 5 m high above water. Since salt extraction ceased here, Selena Todda has been transformed into a magical underground gallery.

 The structures here utilize the only building material that can withstand the corrosive effects of high salt concentrations. The structures are actually the the heritage of hundreds and hundreds of years of experience. 900 years ago, workers began using spruce wood in all their beams, supports, and equipment. They discovered its loose grain structure absorbs salt, creating a layer preventing the wood from decomposing.

Even today, in modern times, we still use spruce wood. It’s the only type of wood that can be used in this environment. [Music] For centuries, workers continued to dig deep bell-shaped mines. However, excavating large amounts of salt from the bottom was time-conuming and inefficient.

 After 800 years of digging down in this way, miners realized that by digging horizontally, the salt could be more easily and quickly extracted. A new way to mine salt was invented. So this is uh the Rudolph chamber. It’s a trapezoid shaped mining work. They are not as deep as the bell-shaped chambers. They concentrate on a larger area, larger surface.

 So this is 99.3% sodium chloride. High quality salt has no color. It’s colorless. Even the walls are transparent. But as they are very very thick, they are kilometers thick, you cannot see beyond them. That’s why they look very very dark because of the lack of light and because they are very very thick. Selena Todd’s salt mines have been at the heart of this region for centuries.

 To this day, engineers continue to ensure this incredible underground space remains relevant. In 1932, salt excavation ended here. But in 2008, this space was reinvented into the world’s first underground amusement [Music] park. Activities here include mini golf and bowling. There are handball and football fields, pool tables, and table tennis.

 There’s also a 180seater amphitheater. Plus the most spectacular attraction, a ferris wheel. This is actually custom made for our salt mine. It was brought here piece by piece and welded together here on this spot. It’s not something you see every day. Standing at an impressive 20 m high, the ferris wheel is a world first. [Music] We’re at a height of about 25 m right now and we are very close to the stellctites.

 The stellctites are very very impressive. These are totally from uh from salt not from calcium or gypson. How long does that takes to form? These tertides form approximately 2 cm per year than 100 years. The size of the chambers was the thing that really surprised me. So, you have no idea when you’re above that underneath your feet, you have an underground world like like this one.

[Music] A second trapezoid chamber was planned. The service tunnel and winch chamber were completed, but the project was abandoned during Romania’s communist era. Years later, the winch chamber was redeveloped for use in a complimentary health treatment known as halo therapy. One of the team at Selena Toida or Solia Colossi is meeting patients who have come here for therapy.

 Some have come a very long way. People who come here for treatment, they usually suffer of asthma or allergies also respiratory problems. Exposure to the salt mines’s microclimate is thought to have beneficial effects on the airways. With high numbers of salt particles here and with very little air movement, there are virtually no allergens to breathe in.

Our son suffered a form of asthma. This mother brought her son all the way from Britain for treatment. Here we didn’t have to use the inhaler at all, which is a really good thing. And we’re hoping that in the future that we won’t need it anymore. Children are encouraged to play in this salty version of a sand pit to expose them to microscopic particles of salt.

 This breathing is much more relaxed and I’m confident that it will help him. He’s amazing here. Even though the mine has been changed into a spectacular amusement park for thousands to enjoy, there are still places only a few get to venture. From the highest point above Teritzia Mine, this location is actually one of our secrets.

 It hasn’t been revealed to the public. We’re about 116 m above. It’s a very unique perspective to the deepest, darkest depths of the abandoned Joseph Chamber, where the remnants of the mining work can still be found. Something no tourist will witness. There is only a limited number of people who have access to this chambers. [Applause] Yeah.

 Visitors testing the echoes from the balconies. Hello. Uh it hasn’t been modernized yet. It’s it’s very interesting to see how a chamber like this looks like when it’s empty and you can really feel the pressure of this large space. Oh, this is very very old. These are nails from the railway system probably 18th century. So this is a bolt from the structure holding up the shafts.

 Probably about 200 years old. People created this this huge chamber and they came here and they worked and extracted all this salt. I can only imagine how long it took them and how hard work it was for them. This is a a piece of wood on which salt has crystallized naturally. So this is probably underneath the water surface for a long period of time for this to happen.

 All the crystals have a cubicle shape. That’s amazing. It’s essentially a a step back into the past. The skill of Romania’s underground enthusiasts means Selena Todd’s past and present have been seamlessly combined for the benefit of generations to come. [Music] [Music] a vast nationwide network of subterranean fortresses. It’s really huge, actually. You can get lost.

Designed to stop a German invasion. The soldiers were shooting blind. I’m proud because they fight like lions. A 2,000-year-old city without water. The water is crucial for the lives of the people. What subterranean secrets are being revealed? It was totally crazy finding something totally unexpected.

 And deep beneath a mountain, a new discovery after 150 years. Wow. This is like a geologist toy shop. And I’ve got all the toys. Beneath our feet lie extraordinary spaces, caves, and tunnels. The span and the size is just crazy. They’ve been designed and built by us. This is the only one with a castle as well as formed by nature.

 But how were they created and adapted? By who and why? You’ve got to face your fears. Throughout history, subterranean life has captured our imagination. Feel so privileged. We’re going further and deeper to unearth the mysteries, the stories, and the secrets of underground worlds. France, a country shattered by many of World War I’s bloodiest battles.

When the war ended, the French became obsessed with being prepared for future conflicts. During the 1920s and 30s, as Germany rearmed and World War II was looming, the French government decided to build the largest network of subterranean fortresses the world had ever seen. The Akenburg is called the monster. It’s the biggest fort.

 It’s huge. What 1930s cuttingedge military technology lay in wait for any potential enemy invaders. They could move the turret 360° and stop it at the exact location they needed to. And what is still hidden from view to this day. This part is secret. In 1929, Andre Majino, a World War I veteran who was now the French Minister for War, convinced the French government to build a defensive line of 142 subterranean forts, 352 gun encasements, 78 shelters, 17 observatories, and 5,000 fortified concrete bunkers along its

entire eastern border. Its construction began in 1930 and was largely completed by 1939 at a cost of 3 billion Franks. This state-of-the-art massive military marvel was designed to deter and slow down any direct German invasion across its border with France. It became known as the Majino line.

 The largest subterranean fortress along France’s 450 km border with Germany is at its most northerly point. It’s called Hackenburgg Fort or Uvage Akamberg. [Music] Hackenburgg is 20 kilometers from the French German border at the highest point in the region. Elise David is an expert on Hackenburgg’s history. The Atkenburgg is called the monster of the Magna line because it’s the biggest fort of all the line.

 Uh it was one of the first to be built. So they put a lot of effort and money into it. It’s more than 80 years old and still here. It’s huge. 1,800 people took 6 years to build the Hackenburgg Fort between 1929 and 1935. This vast 10 km complex of tunnels up to 90 m below ground was built to house 42 officers and 1,040 soldiers. There are two main entrances, one for ammunition, the other for soldiers.

 It’s so large that it has its own train network and stations. [Music] The trains were used uh to carry all uh the tools, the munitions, water, food, all the supplies for the soldiers. And they used these trains only inside. It works with electricity. We have the trolley here with the kenery on the roof and we still use it.

 Around 300 m from the ammunition entrance to Hackenburg is the main ammunition store known as M1. Around 400 tons of ammunition was kept here before being distributed to the 17 combat blocks further inside the underground fortress. So now you are inside the gallery of ammunition store M1. It was used by the soldiers to stock all the munitions in the different cells.

 M1 was the biggest inside this fort and even inside the whole marginal line. It’s really huge actually. You can almost get lost. The M1 ammunition store was built in a U shape. On one side, empty trains would wait to be loaded by soldiers. And on the other side, fully loaded trains with ammunition would be wheeled out of the exit.

 But moving ammunition inside M1 had to be done by hand. The train couldn’t work with electricity inside, which was too dangerous in case of any spark. The soldiers had to push the wagons. They were helped by a winch. They enrolled the cables around here to pull the wagons inside and after to unload the wagons inside all the cells in here. Once inside the store, soldiers used the specially designed ammunition transportation system.

 This is the monorail system. It’s one of the rolling bridge as we call it in French. Overhead Montreal tracks would allow soldiers to lift heavy ammunition from the storage area onto the wagons. Still works. But yes, it works. These are the originals. The same chains, the same Mon. We haven’t changed it at all.

 So yeah, the soldiers touched this and handled these chains 70 years ago and more. M1 was built with safety in mind. Here you have three cells only for the gunpowder. They had fire alarms inside here. They had big tanks filled with water. So when the alarm rank, the tanks uh they were used directly. It was an automatic system to shower all the cells to flood this area in case of fire.

 The entrance and exit to M1 were also built with safety in mind. They’re curved so if anything exploded inside the ammunition store, the blast would be directed out of Hackenburgg rather than in towards the rest of the complex. When the ammunition left the M1 store, it was destined for one of the 17 combat blocks deep inside the 10 km fortress.

 Elise is on her way to combat block 9 in the Western Wing, 1 km from the M1 store. We are still 30 m in the ground at the bottom of the block 9 and the firing rooms are just near the surface. So they needed to get everything up. The elevators were used only for the ammunition cages. That’s why you have a part of the monoral system that goes directly inside.

 The soldiers were not allowed to take the elevator. They had to climb the stairs every time. Same for the officers. There are 143 steps to climb. At the top of the tower is one of Hackenburgg’s big surprises, which is an ingenious feat of engineering. A large counterweight system operates a 163 ton retractable artillery tower.

 It houses two 135 mm howitzer cannons. Should the Germans attack, two soldiers from below could fire six rounds each per minute at a range of 6 km. The soldiers were shooting blind. They were waiting for the orders from the command post downstairs. And thanks to the two observatory blocks, they had all their coordinates.

 You could move the turret to 360° and stop it at the exact location they needed to. They had to believe and trust uh the coordinates they were here just to fire. The soldiers didn’t have any protection for their ears after a while. They used what they had papers, tissues, anything. And even for the two soldiers that were inside the turret, it’s like inside a bell.

 So they had the vibration, the sound, the smoke when they fired. So it would have been really difficult. Hackenburgg was the first and the largest of France’s subterranean fortifications. Its construction informed how many other underground forts were to be built and lived in. 95 km to Hackenberg’s east is France’s second most powerful fort on the Majino line.

Simsofhof is located 4 km west of Bisha in the northeast of France facing the German border. It’s built 30 m below the surface and like Hackenburgg has separate ammunition and personnel entrances. It has its own power utility plant and main ammunition store with a capacity of 33,600 artillery shells that served eight combat blocks.

 Simah took 9 years to build, completed in 1939. On the 21st of August that year, the entire Majino line including Simsov Fort was brought to a state of readiness for war. 876 soldiers were now hidden underground. Betty Brule is an expert on Simsof and how military personnel lived in this subterranean fortress while waiting for the Germans to invade.

 This part is secret. Visitors could not see this part. Here you you feel really how it was because it look like yesterday. soldier bedroom. There was uh between 24 and 36 soldier in a bedroom. One bed for three persons, but they didn’t slept together. They working in shift like in a submarine. So it was hot bank.

 You never could be alone. So you could not relax. If soldiers from outside have to come inside, there was no enough beds. They could made hammock that they could sleep. This was an officer bedroom. And you see there is decoration painting from the wall. It’s beautiful. If soldiers were asleep in the dark depths of Simsof and the Germans attacked, they could be certain of finding their way to their combat blocks quickly and easily.

You see above doors are S exit 30. There is a glowing arrow showing the way of man entrance. So if enemy coming and switch out the light, you just have to following the glowing arrows and you could find the exit. In comparison to the horrific trenches of World War I, living conditions for soldiers in forts like Simsof were significantly better.

 Here they even had a lounge bar which was designed to be a refuge for soldiers. They could pick up mail. They could listening music, play cards, drink something, smoke. And on the wall you have Snow White and the seven dwarfs. This was paint by Roman soldier in the simarov and he decide to paint Snow White because the cartoon from Wake coming in 1937 in France first one in color.

 So very very famous. This is the only room they could relax and feel like at home. Soldiers living in Sims spent much of their time waiting for German invasion. They brought many elements of their lives above ground into the subterranean fortress. This is the chapel. Each Sunday they had holy ceremony.

 In this part of France, religion was very important for demand. On Christmas, the whole crew, 876 persons coming here. They had a special lunch with fuagra, with champagne, with cigar, and they [Music] sing. you know it was uh in German because in this era we speaking [Music] dialect. Finally on the 10th of May 1940 the Germans launched their blitzkrieg against the west.

 Simzahhof and the Majino line initially remained calm but deadly exchanges were inevitable. From the 21st of June 1940, Simsov fired 13,500 shells in just 4 days as it defended itself and surrounding fortifications. You could feel how hard the Sims was fighting. Despite the strength and elaborate design of the Machinino line fortifications, they weren’t able to stop the German troops entering France who invaded via Belgium in the north.

However, the immense firepower and impenetrable design of these major subterranean fortresses meant the Germans didn’t capture a single one of them. I’m proud because they fight like lions. It’s our story. You feel something. Not hear and see, but feel. The French surrendered to Germany on June the 25th, 1940.

 It wasn’t until the 30th of June, and only when directly ordered by commander-in-chief of the French forces that Sims finally relinquished control to the Germans. The underground fortresses that continue to line these borders are a constant reminder of the threat France faced during World War II. Beneath the rolling hills of Darbashir in England, after 150 years of searching, a surprise discovery.

The saw started to plunge into the wall. Wow, that is quite outstanding. Wonderful piece of blue John. Quite exceptional. How has this subterranean space produced a precious mineral found nowhere else in the world? We believe that tree cliff cavern is formed in a rather special way.

 So precious that it’s owned and cherished by British royalty and American presidents. [Music] Darbashier, Britain. The small town of Castleton sits in the northern part of the [Music] county. Vicky Turner’s family have been at the heart of Blue John mining and production here since her father Peter Harrison took over Tri Cliff Cavern in 1945.

Growing up as a little kid, well, I was brought up here. I used to come up on my winch as a baby in a carry cott. So, I’ve grown up here all my life. I can’t remember not knowing about Blue John, not knowing about Treecliffe because it is my life. I grew up here and I am third generation Treecliffe Cavern.

 So, from 1945, the family have been here consistently and we still are. Vickiy’s son John is also part of the family business. I have been working with Blue John professionally for about 6 years, but as a hobby probably for the best part of 20 years. But it’s amazing cutting the stone open, seeing it go from raw to the the finished product as in jewelry or as an ornament.

So, what I’m looking for now is uh I’m looking to keep the stone right up against the silver, creating a nice dome. And that looks like we’ve got no flaws. We’ve got a flawless pattern stone. One of my favorite bits is cutting the stone open. Doesn’t matter if there’s clay covered on the outside. Once you’ve cut the stone open, it’s clean.

 You can see all the patterns, all the colors just revealed to you. It’s amazing. [Music] Blue John was formed around 300 million years ago when this landscape was buried 3 km beneath the Earth’s surface. Liquid minerals at a heat of about 120° centigrade were deposited and crystallized as veins in the cavities within carbonifpherous limestone. Dr.

 No Wallally is a geologist and expert on blue John. Blue John is calcium fluoride which is a mineral called fluorite. Despite much investigation, geologists remain uncertain of the origin of blue John’s color. One hypothesis suggests that atoms in the fluoride molecules may somehow have been disturbed to create this unique color.

We believe that the blue color here has been produced as a consequence of the irradiation of the layers of fluorite within the cavities. The origin of Blue John’s name is also shrouded in mystery. Some believe it comes from when it was exported to France in the 18th century. The colors of the mineral blue and yellow when said in French being blon.

Wow. Well, this is like a geologist toy shop and I’ve got all the toys. Wonderful piece of blue John. Beautiful. That is quite outstanding. I can see that this material is going to be quite exceptional. It is believed the Blue John was first discovered by accident in the mid 18th century.

 Gary Ridley is Treat Cliff’s cavern manager and is a Blue John miner. The original miners that discovered these caves were actually lead miners looking for lead. And when they broke through into these caves, they found very little lead at all. Nowadays, we can see just these empty passageways. Back then, they would have been absolutely full of Blue John.

 I would imagine for the first few years he was hardly doing any mining, just be picking the stuff off the floor around them. So much blue John there would have been in these in these caves. In 300 years of mining on Tree Cliff Hill, about 2,000 tons of Blue John has been extracted from 15 so-called veins.

 They all vary in color and form. Most of the Blue John veins are are very different from each other. And nine times out of 10 when you see a piece of uh blue John jewelry or an ornament, you can tell exactly which vein it’s come from just by looking at it. Treat Cliff Caverns and its Blue John deposits only run about 150 m into the limestone rock.

 Mining here has never been on a large scale, requiring explosives. Each piece of Blue John has carefully been extracted in the same way for centuries. The mining methods would have been called plug and feather. And until quite recently, we used to still use the plug and feather method. You would had two miners and one would have had a a drill bit, quite a long drill steel, and he would have held it on his shoulder and the other miner would stand behind and he’d whack the end of the steel and every time he whacked it, the guy in

front would twist the steel. And this must have taken absolutely hours to drill these holes. The busiest period of blue John mining was in the 18th century when about 20 tons per year was extracted. Once the blue John industry had taken off, of course, everybody wants to get in on the ice, especially if you’re a local.

 So you could walk the hills and you might find some of this vein uh coming out on the surface in little outcrops. So you could uh dig some of the blue John out from there. Probably at its height, there would have been as many as 20 little operations going on on this this one little hillside. Today there are only two working Blue John mines.

When Treat Cliffe was taken over by the Harrison family in 1945, the previous tenant, John Royce, who was unwell at the time, showed Peter Harrison around the witch’s cave and suggested he had discovered an unmined part of the upper tree cliff blue vein. He was told by John Royce that there was a vein of stone that he wanted to to dig out.

But John Royce died before Peter found out anymore. Granddad was never told whether it was a vein still in the the wall or loose rock ready to be carried out. Despite huge efforts, Peter couldn’t find the mysterious Blue John deposit. It became known as the Lost Vein. But then, 68 years later in 2013, Gary Ridley was showing Peter’s grandson John how to search for new veins using hand tools on the soft clay.

 when they stumbled across something exciting. About half an hour into digging, I discovered this vein in the wall, which was it was telltale signs of hidden vein. There was carpet, sticks, stones to hide it. They rediscovered the lost Blue John deposit. He hid it and he hid it a little bit too well. And yeah, we had about four or five tons of stone out of it.

Two years later, in 2015, Gary was testing out a new way of mining using a stone chainsaw in an area of the cavern he’d never dug before. At first, I was drilling into limestone. Uh, it was quite hard to drill and the water that was coming back from the chain was very white, so I knew we’re in limestone.

 And then after a few minutes, the color of the water started to change and the saw started to plunge quite quickly into the wall. So I knew we were in Blue John. Unlike the discovery of the lost vein, Gary had found an entirely new variety of Blue John and the first for over 150 years. I then thought, well, how much is this? We brought a jet wash in.

 and we start to clean the walls down and then we could see that the vein itself is traveling into the rock and it was twisting up around the corner. So, there’s a fair bit to go out and it should keep us going for quite a few years. And once I got a piece outside and cleaned it up, we realized what we got was a completely new vein.

 And uh Vicky, the owner, decided to name it after myself. Having a vein named after myself, it feels a bit weird at times, but keeps your memory going long after you’ve gone. I suppose so having that knowledge is quite uh satisfying. Gary’s meeting geologist null Wally to show him an example of his Ridley vein. Beautiful piece of ridley vein with particularly characteristic yellowy orange color alternating with the deep blue and purple.

 You can see there’s a lot more yellow in this vein than the other veins as well. And we’ve got these two close bands about halfway down. The first time it’s seen the light of day in probably 300 million years. The Riddley vein is the 15th to have been discovered and only the second to have been named after a person.

 Blue John and Tricliffe Cavern aren’t just of interest to geologists. Professor John Gun from Birmingham University is a speediologist, a cave scientist. He’s fascinated by how these caves were formed. We believe that tree cliff cavern is formed in a rather special way not by water descending but by water actually coming up and we call this hypogenic spelioenesis.

 So the water rising up is forming the cave. The very earliest phases of developing a cave in this area occurred over 300 million years ago. Then we had a phase when the rising fluids were mineralizing fluids and they brought in the fluite minerals, one of which is blue John. John is here to explore a recently discovered series of caves named the hidden depths.

 He wants to see what traces of Blue John there might be in this newly found space. He’s agreed to film his journey down through the rock with a small camera. There was always a thrill in going into somewhere that very few people have been in, some newly discovered cave. From a scientific point of view, I was particularly interested.

There is no question about it. Those chambers down there are hypogenic. You can see uh that there’s been rising water in [Music] there. They’re also really interesting because they’ve got the blue John mineralization. [Music] So this is some of the lowest elevation blue John mineralization we’ve got in the area. Great.

So it’s absolutely pristine down there. Absolutely beautiful. You know, one of the uh really amazing things about a place like this is how few people have been down there. And we reckon that less than 20 people have ever been down in that part of the cave. More people have been to the moon. [Music] Less than one ton of blue John is mined from Treat Cliff Cavern every year and there’s no need or intention to prospect for Blue John within the hidden depths for the time being.

 The demand for Blue John as an ornamental stone diminished throughout the 20th century, but with the turn of the 21st century and the new veins being found, it’s experiencing something of a renaissance. It’s a great vein to work with and the pattern really stands out no matter whatever you put it in. There are very few areas in the British Isles that you can see geology so clearly displayed.

 This is a a superb example. [Music] People like John Turner are working with the mineral a new and large old vasees have realized enormous prices at auction houses. Blue John is rare enough that it will never support a major industry, but its role within the decorative arts continues to evolve. I’m 30 now and you know I’ve been running around here for the last 11 years like a child. Loving it.

 Love the [Music] place. Istanbul, Turkey. A transcontinental city straddling the Boserous Strait where Europe and Asia meet. For more than two millennia, its location has been strategically important. But without one thing, it couldn’t survive. Water is the key strategic point in the surviving of the city and crucial for the lives of the people.

 And there is no uh freshwater sources. What extraordinary feats of underground engineering meant this city could thrive for nearly 2,000 years? I’m looking at it like, “Wow, how the hell did they think about this?” We’re sitting on top of history. And what subterranean secrets are still being uncovered today? It was totally crazy finding something totally unexpected.

In the year 395 AD, Istanbul, then known as Constantinople, was the capital of the Baantine Empire. But this was a city with no fresh water source. With people flocking here from all over Europe and Asia, an ingenious solution was needed. In the 6th century AD, during the reign of Emperor Justinian, water was transported 19 km by a series of aqueducts from the Bgrade forest and stored in underground systems in the city center.

 Over the centuries, as the city is being redeveloped, Istanbul’s hidden past is being revealed one system at a time. When redeveloping an old site for his latest shop in 2004, Jenk Tutin of a local carpet store got more than he bargained for. There were three houses, double floors above. the excavator hit and that was one of the doors that led us under those houses.

 Thanks to the excavator, that’s how the whole thing started. [Music] Jenks builders had uncovered a subterranean secret. This whole place was covered up with dirt and water and a bit of everything. It was totally crazy finding something totally unexpected. What they’d found was a system about 1,700 years [Music] old.

 Jenk’s carpet shop is on the site of a very important ancient palace. This was the water for the upper class. They were using it to shower. They were using it to cook their meal. They were using it to drink it. This system and its 18 marble columns would have provided water to the great palace of Constantinople during the reign of Emperor Justinian.

 Can you imagine the water that was that was preserved in here was actually drank by the uh royal families. He has invited Dr. Karum Aluk to his carpet shops restored subterranean secret to see if he can reveal anything more about it. Dr. Kum is an archaeologist and historian and has been researching Istanbul systems for more than 20 years.

This is the original staircase. You can still still see the the the steps. Yeah. So that was the entrance. What’s the stone made out of? This is a brickuilt structure. Okay. Brick. Brick and mortar. Okay. You can see a thick layer of plaster. This is the waterproof hydraulic plaster. Original baantin plaster.

 When the plaster ends, this indicates the maximum storage level for the sister. These are monolithic marble columns and these are reinforcement built during the later period in the Ottoman era. Okay. So this is not the actual original one. That’s the original one. That’s the original. Exactly. Fantastic. I find this system beautiful.

 To me it’s it’s an art. To me it’s history. I’m looking at it like, wow, how the hell did they think about this? And what made them do this? But whoever did it, I’m happy because this is history. Beneath the city of Istanbul, there are reportedly a staggering 200 ancient subterranean systems. Professor Fareradan Osgumas is an archaeologist and Baantine expert.

 He was charged with restoring the oldest system ever discovered. It’s called Theodosia [Music] system. 2009 I was appointed for the restoration consultant of this substructure. It’s very important for us because this is one of the few Byzantine substructures very well preserved to our days.

 Theodosia system was discovered under a 20th century mansion and a municipal building that was constructed in the 1950s. It is 45 mx 25 in size and has 32 marble columns 9 m in height. So, how were these robust subterranean structures built? Before the construction, engineers dug the ground and all the height and the width and the length of the structure fixed by the engineers and then they put these column shafts.

The walls of systems were typically made from brick 4 m thick. Builders then needed to guarantee they were watertight. They plastered the walls. Those plaster is waterproof. And if you look at the corner there, you can see the corner is rounded. If the corners of systems were not like this, the water itself could go out of the system easily through the joints or the walls.

 When we excavated big empty space like this, if we see the corners surrounded, then we did we assume that this must have been a system water system once used in antiquity. The 8-year renovation of Theodosius has been true to its original design. The plaster on the walls, the restoration plaster from 2016, it has the same ingredients in it.

Lime and ground bricks. That’s it. The lime gets solid in 15 days, but it can stand 5,000 years maybe. If you cement, it gets hard 3 hours, but that destroys the fabric of the wall because it has salt in it. That one has no salt. Across the ceiling are holes leading to the surface above, meaning water was available to everyone.

Those holes are used for taking water by buckets uh from the system. Some of the systems have uh fountains outside the building and uh this was the main source of water uh for the public use. The people of Constantinople had perfected the design and construction of systems. There are remarkable similarities between all of them.

 Those are original steps from the fifth century belong to that system hidden underneath this modern plaster. All of the systems in Istanbul from the Byzantine period have steps on one of the walls of them. Those steps may have been used for cleaning purpose, but we don’t know for sure their original function. What is clear from the design and construction of these 1700y old structures is that they were built to last even through natural disasters.

 Although many earthquakes hit this city and a lot of bu buildings were damaged badly, some of the ancient buildings including this one remain to our days almost without any damage. So this tells us that good engineering, good calculation, good workmanship, thanks to all those three things, uh beautiful ancient buildings remain to our base without any damage.

The most famous system known to have survived beneath Istanbul is known as Basilica system. The Basilica system is is the biggest of the systems in in the city and it’s it’s an enormous engineering achievement. It was built in the sixth century to supply water for the great palace and the nearby buildings during the reign of Justinian.

 It is very sophisticated system of water supply. It’s definitely one of the most important uh monuments of Baantine Constantinople. This vast cavern measuring 138 m by 65 m could be filled with 80,000 cub m of water. Its ceiling is supported by 336 marble columns, each measuring 9 m high. Many were recycled. Here we can see one of the reused architectural elements.

 Here a stilized wood trunk manufactured in 4th century. The decoration on this column resembles others found elsewhere in the city. Ancient texts suggest that the tear-like decoration pays tribute to the hundreds of slaves who died during the construction of the basilica system. In the northwest corner, there are two striking columns standing on the heads of Medusa.

 That’s probably from the 4th century Roman. There’s a second one you can see which is upside down. It’s not known where these came from, but the significance of their use and orientation is clear. In a newly Christianized city, characters like these that were associated with previous belief systems would have been scrapped. These are also reused in the structure.

A local tradition also suggests that the faces were positioned sideways and upside down to eliminate the petrifying power of Medusa’s stare. Basilica system is one of a kind and biggest in in here in this land. There’s nowhere we can have a bigger example of reserve and structure like this.

 Systems aren’t needed for water supply today, but experts and passionate locals are determined to make sure they’re preserved for generations to come. This had to be preserved and had to be um refurbished. We’re trying to provide people the information and the history that they’re actually stepping on. Cheng and his colleagues have taken extraordinary steps to ensure that Nakas system is preserved in order to protect and presume the whole beautiful history under this building.

 We have a huge steel construction that carries the weight of the main building and this way doesn’t give any pressure to the sistn itself. The weight is pushed out. Many of Istanbul’s subterranean systems have been transformed, giving them relevance to life in the 21st century. Most of these things are privately owned.

 One of them is a restaurant of a beautiful hotel. Some of them are museums. Some like this is exhibition hall. We’ve had painters from around the world coming. We had musicians. There are a lot of things to be discovered in stumble. Most of these buildings in stumble have mysteries. We’re sitting on top of history. You know, if we drilled at the moment on these roads over here, we’d probably find more sistns.

There is no other Roman city to compare with this size and number of the water storage units as Constantinople. [Music] [Music] in a mine fraught with dangers. They had gas explosion fires lasted 6 months. Why did the miners create these amazing underground cathedrals? Miners were very religious. They had chapels under the

ground to pray for safety. How are these life-saving shelters from World War II? They were amazing engineering construction. Turning into life support for the future. What goes around comes around. And what is the amazing history of the underground world hidden behind this castle? We have two Slovenia. One Slovenia on the surface and other Slovenia under the ground.

Beneath our feet lie extraordinary spaces, caves, and tunnels. The span and the size is just crazy. They’ve been designed and built by us. This is the only one with a castle as well as formed by nature. But how were they created and adapted? By who and why? You’ve got to face your fears. Throughout history, subterranean life has captured our imagination. feel so privileged.

 We’re going further and deeper to unearth the mysteries, the stories, and the secrets of underground worlds. Poland lies in the center of Europe, east of Germany. It’s home to 38 million people. Best known for its iron and steel industries, Poland’s most famous historical export was salt, a mineral once considered more valuable than gold.

The income from only this mine provided 25% of all income in the state treasury. But while many mines can be dark and spooky places, how did this one become such an incredible space filled with music? For me, listening to the brassband playing so deep under the ground is magical. What hazards did the miners have to overcome? Chambers started to collapse and every collapse and the mine make sink holes outside to create this world of wonder carved by hand.

 There is nothing like this in the world. [Music] The Velichka salt mine lies on the outskirts of Kkow in southern Poland. It opened in the 13th century and for the next 700 years the miners dug down nine stories to a depth of 327 m. There are 250 km of tunnels and more than 2,000 chambers. and camel Bronovski, a chief foreman, knows their layout intimately.

Salt was taken out and empty space was left behind. What distinguishes Vilichka above other mines is that once the salt was extracted, the empty voids were transformed into extraordinary spaces, a cathedral, four chapels, and galleries of statues and sculptures, all carved from salt by the miners.

 Chambers were not created for the purpose of having an underground room, but they were more like a byproduct of the excavation process. The mine was built to exploit rock salt deposits that were created around 13 million years ago when a prehistoric sea evaporated. Petritia Antonak is an expert on the history of the mine.

The rock salt you can see here is the primary salt. The dark rock comes from the ancient sea. It is still possible to find fossils trapped in the rock, fossilized remains of plants and animals. Before the mine was built to process prehistoric rock salt, local people had found the mineral rising to the surface in pools and lagoons in the local countryside.

For many centuries, the local people produced salt by evaporation of brine, salty water. When the sources of the salty water on the surface were almost exhausted, they started digging, sinking wells, hoping to find brine under the ground. On the bottom of one of them, they found rock salt.

 The first lump of the gray rock salt. What they found is halite, a crystalline form of sodium chloride. This is a salt crystal, cubic in form. Salt crystallizes in cubes always. And the salt crystals are perfect cubes. In the past, salt was very, very expensive. 600 years ago, 1/3 of the Polish state income came from salt. 36%.

Salt was used to preserve food. That’s why it was very, very precious. There is nothing like this in the world. The mine grew quickly. Working in extremely challenging conditions, men used basic hand tools to dig into the unknown for the precious salt. The mine employed over 300 workers and produced over 7,000 tons of salt per year.

 In the Middle Ages, the salt from the village salt mine was considered to be the best salt in Europe. The miners uh produced blocks of sold way over a ton. By the 16th century, the miners were winching out two-ton blocks with the help of horses. At any one time, there would be 100 horses in the mine, and many of them spent their entire lives underground.

Gr. So, there were stables on different levels. The miners would bring food for them, hay, oat, water to drink. In the 18th century, the mine produced about 30,000 tons of salt a year. And at the peak in the 20th century, more than 250,000 tons a year. Falling sold prices meant that commercial mining stopped here in the 1990s, but the end of the industry coincided with the rise of tourism.

 UNESCO made the mine a world heritage site in 1978. Now visitors from all over the world marvel at the chapels and sculptures created by the workforce in their free time. The miners were very religious. They had chapels under the ground to pray for safety. And the chapels were illuminated better than the other chambers.

 There are 15 chandeliers around the mine. Their frames are made of wood and decorated with strings of pure rock salt. In a painstaking process, the sculptors shape them, giving them a beautiful glass-like finish. The largest and most stunning of all is in the Chapel of St. Kinger with 2,500 crystals. Before a service in the chapel, a minor would lower a chandelier on the rope to light candles.

The chapels contain carvings which are almost a century old. Jesus and his disciples at the last supper took 9 years to carve from 1936 to 1945. It’s inspired by Leonardo da Vinci’s Renaissance painting. Elsewhere, there are chambers with carvings of everything from gnomes to local heroes.

 The Januit’s Chamber has a setpiece tableau called The Great Legend, carved in 1967. It’s the story of a Hungarian princess who became a local saint. Princess Kinga came to Kraov in 1243. Now, the legend says that when she was about to leave Hungary, she asked her father to give her something as a wedding gift for the Polish nation.

 At the time in Poland, we had no salt mines and salt was very expensive because it was used for food preservation. Hungary was rich in salt. They had many salt mines. So her father, the king of Hungary, gave her one of the mines. It’s believed Princess Kinga dropped her engagement ring into the mine which was later found in the first lump of salt extracted from Vilichka.

 Uh salt was produced in the Zarya since the Neolithic period. So for over 5 and a half thousand years, but excavations began in the 13th century. So more or less at the same time that she came to Kraov. The tradition of carving statues from rock salt continues to the present day in the mine’s ground level workshop. This is uh Tadushko, one of our national heroes and his soldiers.

 And here a minor from the middle ages. And on this side here we have a symbol of the mine. The tools the miners used to excavate the chambers hundreds of years ago. A peacock and a mallet. And on the top there is Polish eagle. The whole statue is made out of the rock salt from the mine. However, the crown is made of the purest salt similar to the crystals used to decorate the chandeliers for example.

Not only did the purest salt in the mines find its way into statues, it could be crushed and refined by hand, ready for use in the home. Some workers were called salt crers. They were supposed to crush salt to the powder with these bats to produce fine salt, table salt. It is really hard. You don’t need to clean this salt.

 This is eatable salt. You can normally use it for cooking, preserving food. It’s like table salt. This is what happened with salt. When you crush it to the powder, it’s not gray. As the mine grew over the centuries, so did the risks involved in maintaining this enormous subterranean world. [Music] They had gas explosions, fires, and shaft collapses.

Once in 1648 the woodcut fire from the torch which started big fire in this mine. The fire lasted six months. After the they cut off the fire they couldn’t start working for next year because of smokes and dangerous gases. Chambers started to collapse. Big blocks of salt fell down to the ground on both sides and every collapse and the mine make sink holes outside at the same time.

 So it’s also serious problem outside. Another hazard was methane gas. When it reaches levels of more than 5% concentration, it becomes highly flammable. The smallest spark would cause it to explode. They employed special group of workers called gas burners. So gas burners were always sent first to the mine and they burned gas with torches on sticks and they were looking for the gas at the ceiling because gas is lighter than air and goes up to the top of chamber.

 So they had to crawl on the floor or walk on their knees to avoid u the gas explosions. But they risk their lives every time. So it was a really dangerous job. These people wanted to do it for good money because it was also very well paid job. During the early 19th century, miners were issued with the revolutionary Davyy lamp.

 The design prevented the risk of explosion and had other benefits too. They didn’t give much light, but they were able to detect the presence of gas. The the flame inside changed the color and informed miners about the dangers. The lamp flame burns inside a screen of wire mesh. The mesh lets in the air needed to keep the flame burning as a light source, but stops it from igniting flammable gas in the tunnel.

Uh, it was 1825 when they started using Davis lamps as well as methane gas. Another risk to the miner’s safety was water. To prevent the risk of flooding and collapse, the miners built channels to drain the water away to pools and lakes deep in the mine. Uh wooden halfpipes were mounted and then using gravity, water was directed from the point where the leakage accured to one of the shafts of the mine.

 Wood was also used to support the largest chambers and the ever growing collection of salty chandeliers. Whereas metal would have corroded in the damp salty environment, the wood turned rock solid. For centuries, wood has been used to support ceilings of chambers in salt mines. Salt preserves it. The oldest wooden constructions we can find in the mine are 400 years old.

 The risk of roof collapse was a constant danger. Certain types of timber were useful to the miners because they would make cracking sounds under the strain. They would serve as an early warning system. There is about 2 million cubic meters of the wood in the mine. Uh the first miners used a lot of pine and spruce to build the constructions as they make a lot of noise cracking.

 Using wood which makes a lot of noise cracking was uh the first alarm system of the mine. At the center of this giant timber web lie chambers which are now used to host numerous cultural activities. The brassband in the mining village started in 1830. It’s a tradition. Almost every Polish mine has a brass band. The band was formed to entertain rich sightseers such as the Austrian emperor France Joseph I in 1851.

They played this popular Austrian tune, the Redetski March, in his honor. For me, listening to the brass band playing so deep under the ground is magical. [Music] After 700 years of industry and craft, the miners of Ilichka have left behind an incredible legacy and one of the most beautiful man-made underground worlds in Europe.

 The UK’s capital, London, home to more than 8 million people squeezed into just 1,500 km. Beneath its busy streets lies a network of tunnels made of steel and concrete constructed to protect its citizens from the dangers of World War II. Here’s the entrance topped with 5 ft of reinforced concrete. For years, this subterranean structure has been left untouched.

Down here is a hidden world, and there are artifacts here still from its original use. What remains of these huge subterranean facilities? 74 years later I think that provided refuge for a generation of Londoners. So welcome back to the Clappam deep level shelter and how a visionary farmers reusing this amazing space to enhance the future of food production.

 We can create the optimum environment for growing get up to 60 harvests a year. Britain declared war on Germany in 1939. The conflict raged for 6 years and changed millions of lives forever. While most of World War II was fought in Europe and the Pacific, the German air force, the Luftvafer, brought death and destruction to the streets of Britain, especially in London.

 Throughout 1940, the Germans bombed heavily many cities, but London in particular. Um, and the people really needed shelter from that threat. Here in Clappam, South London, subterranean historian Martin Dixon is about to enter an underground world that few Londoners are aware of. The country was desperate to find accommodation to shelter the public.

 By 1940, they decided they needed a radical solution and as a consequence, they built eight deep level shelters for the general public. Each was to house 9,600 and this at Clappam Common is the entrance to one of them. From 1940 to 41, eight deep level shelters were built underneath London.

 There were four north of the rivers and four south partially with the idea that postwar they could be linked together into an express um rail service underneath the capital. The Clapam Common Shelter is hiding in plain sight right in the middle of a South London high street. Here’s the entrance uh protected by a solid pillbox because the shaft was the most vulnerable part of the network topped with 5 ft of reinforced concrete.

There were attempts to make the exact location secret, I guess, to make sure that they weren’t specifically targeted by German bombing, but in a sense, having a secret shelter is pretty pointless because you want, you know, the top side of 80,000 people to to actually know where they are and be able to efficiently enter.

 On hearing London’s air raid sirens, thousands of people would have raced for cover in these deep tunnels, always at short notice. [Music] People say that that rather than being frightening to go underground, it was very reassuring. And when you see the photos of the the damage that that came as a result of the blitz, you can imagine that uh that coming underground and hearing um the the sort of reassuring rumble of the Northern Line was actually quite a a comforting place to be.

 Um and in general, if it’s warm above ground, then it’s cooler here, and if it’s cool above ground, it’s warmer here. So it was probably quite a pleasant temperature as well. So in less than two years in the middle of a major war, they were amazing engineering construction. The shelters lie 30 m below the surface. That’s twice as deep as the London underground tube system.

And at this level, shelterers would be safe from any conventional bombing. Each shelter was split into two levels by a mezzanine floor. To get into the shelter is almost 200 steps. Um, and although there’s a lift, that was only really available for supplies and for catering. All of the shelterers would have to use the spiral staircases to get into the shelter.

Getting underground quickly was a matter of life and death. These ingenious stairwells layered two separate staircases within the same shaft. Because there are two levels to try and increase the speed with which the shelterers could enter. There are actually two spiral staircases interled. Um this one leads to the lower floor and the one above eventually leads to the upper floor.

 Skilled tunneling experts from all over the UK were brought in to construct the shelters. Mostly was hand dug, done largely by miners because they had the skill in excavation. Each shelter took a year to dig out. The miners started with the two vertical shafts which now contain the stairs, then cross passages, and finally the main shelter tunnels.

 It’s the same technique used to build a regular tube station just further down. For every advance in the shaft um that the miners would insert um one more segment, therefore protecting themselves and the structure from collapse. As it was extended downwards, these two foot wide segments were inserted to provide stability for the whole structure.

 Not only did the miners employ the same tunneling techniques used to build the tube network. They also used the same cast iron construction materials. Virtually all of the um tunnels aligned with what you might regard as standard London Transport um tunnel linings. and LPTB, London Passenger Transport Board, shows that it was a standard London Transport segment reused for the emergency war work.

 So they were, if you like, diverted from from from tube construction and used for this for this shelter. 100 m inside the shelter, there are still signs of the working infrastructure. So this looks like the entrance to the original plant room. Yep. Down here would have been all the ventilation gear to make sure that there was a supply of clean air for the shelters.

 There would have been electricity and also water and waste water in order to provide those necessities. The total length of each site is about 400 m 420 m. So collectively taking it end to end there was about a mile of shelter space within each structure. Bunk beds were built along the walls. People were expected to bring their own bedding.

 And then on top of that there was the facility for toilets for catering uh first aid or medical posts and so on. Given the size of the Clappen Common Shelter and the seven other deep shelters in London, it’s remarkable how invisible they are to citizens today. So up above we’ve got the tube train rumbling away and further up we’ve got Clappam High Street which is a busy thoroughare.

 Thousands of people passing and repassing. Yet down here is a hidden world. Northern line trains pass overhead every few minutes carrying thousands of passengers over the shelter. to go right up there through that brick wall. That’s Clappam common tube station. When work began on building the shelters in 1940, the Royal Air Force were slowly winning the battle of the skies.

 The Luftvafer conceded defeat after losing more than double the number of planes than the British. Two years later, when the shelters were finally ready, it seemed as if they might not be needed. After all, the immediate threat uh the German air attacks had had reduced somewhat. The other reason is the government still had the concern that if people went into this sort of shelter that they developed some form of I think they called it deep shelter mentality, meaning that they would retreat into themselves and that that it would take away some of their fighting spirit if

you like. Within two years, the Germans had developed new weapons in vengeance for Allied bombing raids on Germany. In June 1944, the Germans launched their attack of of the VW weapons. Uh, first off, the the V1 flying bomb or the doodle bug. Seeing the devastation that that could potentially cause to London, the government did decide to open the shelters to the public.

 New deep shelters built for this emergency were put to use and saved the lives of thousands of people. There are very few Londoners still around who remember the experience of racing down into the Clappam Common Shelter. But 92year-old Joyce will never forget these wartime experiences. [Music] I remember my father coming in and saying, “We’re at war.

” Londoners grew to fear the V1 flying bombs. When they got those buzz bombs, you’d hear the noise of the engine and then when it stopped, you knew it was coming down. [Music] Hundreds of homes, hospitals, and public buildings were hit. Joyce’s family home was damaged when one of the V1 bombs exploded nearby. The front door was blown right by down the passage.

 For the first time in 3/4 of a century, Joyce is making her way back to the Clappam Common Shelter where she once took refuge from the Nazi buzz bombs. And Martin’s going to show her around. Joys, it’s lovely to meet you. Tell me what it was like living in London during the war. Well, it was very hectic. You never knew if you were going to be safe.

 When you um stayed in the underground shelters, what was that like for the first time? Oh, wow. It was quite good. You got a good night sleep. Yes. Yeah. Should we go inside now? Yes. Does this seem familiar coming back after all these years? Yes, a bit. In those days, were you allowed to use the lift? No, I had to walk down the stairs.

Someone told me it’s 180 stairs down. There we go. Right. Well, 74 years later, I think so. Welcome back to the Clappam Deep Level Shelter. Oh, it’s a relief to get away from the planes and Yes. guns. When you were down here, how did it feel like going back to the surface in the morning? What What were your feelings then? Was her home still there? I guess for some people the homes would have gone. Yes.

 Since the war ended, few uses have been found for these old deep level shelters. But now, part of the Clappen Common Refuge bears a passing resemblance to a sci-fi movie set. If the people who built this could see what it’s being used for today, I I don’t think they’d believe it. 15 m below the Northern Line, there now lies an underground world that’s dedicated to growing food.

What an amazing transition from World War II to space age cultivation. The new use for this World War II air raid shelter could herald the beginning of a new era in food production. Bur underground is an urban farm. It’s situated 33 m underneath the streets of Clappam, London. We use hydroponics LEDs to produce micro greens, which are tiny herbs packed full of flavor.

 This installation is at the forefront of an exciting new global trend for underground farming. And initially we looked at overground spaces, but the underground worked really well for us because it’s an insulated space and we have 100t of soil above us. We’ve got a consistent temperature about 15° year round.

 Um, we can create the optimum environment for growing. Fresh herbs are in demand all year round. Many have a large carbon footprint because they are imported by air and road and are highly perishable. This farm delivers to London’s food markets just a few miles away. This product conventionally comes in out of season from places like Israel or Egypt.

 So, it’s reducing food miles uh and pollution from large vehicles coming into the capital. Um and in turn, this actually reduces food waste by giving our customers a longer shelf life. This shelter once protected the vulnerable. Now it’s being repurposed to nourish the growing urban population. Is this the future of farming? We power the site entirely by renewable energy and we’re working towards carbon neutrality.

How we are going to um use waste products like CO2 or um gray water really sort of resonated with me. Other countries including the USA and China are also looking at ways to repurpose their disused subterranean spaces and for ways to feed their ever growing populations. It’s phenomenal to know how much space is available about how future cities are going to feed and power themselves with this growing population that we we expect to have an extra 2 billion people on the planet by the middle of this century. These deep

shelters came about through resourceful planning and saved the lives of thousands of Londoners in their time of need. I think it’s somehow nice that what originally was was beds for people has become beds for produce. So what goes around comes around. This shelter could be improving our lives for many years to come.

There’s a lot of locals that perhaps uh don’t even realize this big space is underneath them. [Music] Slovenia, a mountainous country bordered by Austria, Croatia, and Italy. Its modest land mass contains no less than 39 peaks, and beneath them, a region of limestone gorges, caves, and underground rivers known as a cast.

 For centuries, the Slovenians have relied on this magical underground world for protection from the elements and from invaders. But who was the first to chart these subterranean passages as a means of survival? We can say that Arasmos was the first cave explorer in Slovenia. What underground secrets are hidden behind this incredible castle perched on a mountain side? They got water into the castle that was fresh, clean, and completely safe.

 And who were the experts preserving and mapping the caves for future generations to enjoy? In Slovenia, we say that we have two Slovenia. One Slovenia on the surface and other Slovenia under the ground. Pgerma Castle in southwest Slovenia is the starting point of a 14 km subterranean system in an area which boasts four stunning caves eroded by underground rivers.

 The first of these breathtaking chambers is a fantastic secret hidden away behind the 16th century battlements of Pjama Castle. This impenetrable fortress which covers the cave mouth is perched on a cliff face 123 m high. Saya Kades knows the fascinating history of why it was built and what it conceals. It is officially the biggest cave castle in the world.

 The caves were first fortified by a medieval religious order before passing into the ownership of the house of Hapsburg, one of the great royal dynasties of Europe. They built a castle here to make this road safer. The underground site was ideal because of its strategic position on an important trade route, linking the central European heartlands with the Adriatic Sea.

And one of the inner walls of the castle is a natural wall. It’s much easier to defend the castle from just one side. It’s also perched in the middle of this vertical wall. It was pretty much impossible to enter the castle from any other place but through the main door. If you go to the top of the cliff, it’s completely hidden as a safe place. This was always a perfect choice.

Starting from this crevice deep in the limestone cliff, new stages of fortification were added step by step over three centuries. Each improvement embellished the caves with layers of secrecy and security. The last part of the castle was added in 16th century. So, every century added something new. The earliest castle was a confined and damp space.

 Hardly the most comfortable place to live. When rebuilt in the 16th century, ingenious craftsmen came up with an early kind of cavity wall system with voids of air between man-made and natural rock faces. The air pockets provided insulation and the double walls effectively became a warren of hidden roots leading back into the caves. There was a theory here that people of this castle have a system of secret passages so that they could move from one part of the castle to the other without being seen from [Music] outside. The defensive and strategic

value of these caves was exploited by their most famous resident, the knight Arasmama, better known as Arasmus, a rubber baron who lived here during the 15th century. Following a dispute with the Austrian emperor, Arasmus was sentenced to death, but he escaped into the cave system.

 But eventually, the soldiers that the emperor sent found him and began with a siege. Now, he knew this castle very well, so he knew that he’s going to be very safe here. The caves offered Arasmus one big advantage, a natural supply of fresh water. Because it drips through the poorest rock, it’s safe from contamination by any attacker. The ingenuity of people building the place is apparent.

 You have dripping water in any cave. They took advantage of what nature was already offering them. And here we can see the channels. This is this is part of the water supply system here in the castle. That’s how they got the water from above, from the cave to the lower parts of the castle where it was needed the most.

 They got water into the castle that was fresh, clean, and completely safe. It was impossible to to poison it. Arasmus may have had a continuous supply of clean water, but he needed food. The night Eron had a cunning plan which depended on the caverns behind his stronghold. At the back of the castle, in the cave behind, Arasmus had a secret passage.

Now, the secret passage comes out in the forest above the cave. It is impossible to find it from outside. Arasmus would use a secret passage to get food supplies into the cave. The secret route was unknown to the emperor’s soldiers. Arasmus used it to avoid their siege and continue his raids on neighboring towns.

With the castle fully supplied, he would taunt the soldiers by sending food down to them. Kind of showing them that there’s absolutely nothing they could do to him. They had no idea that it would be possible to have a secret passage in a solid rock. So for them, according to the legend, a much more logical explanation was that there was some dark forces on his side.

 Of course, Arasmus wasn’t a magician, just a devious robber baron who made capital from these extraordinary and beautiful caves. The most important part about that passage is that it’s not just a cave that it was a lifeline during this siege. The legend of the secret passage has been a lifetime inspiration to cave explorer Mayan Villar.

 I was impressed when I was a boy here looking at this big cliff and this beautiful castle. When I start with cave exploration, the cave under the castle was my first cave that I visit. We can say that Arasmos was kind of a first cave explorer in Slovenia at that time. They were using candles, maybe some oil lanterns, and this was it.

 It was a big challenge for him to go out. He was kind of a a revolutioner at that time for caving. Now, Mayan retraces the difficult climb that Arasm once took by candle light. It’s a stunning cave network eroded by fresh water over millions of years. The secret passage is almost vertical and extends for 42 m from the rear of the castle cave up to the forest above the cliffs. Dimensions narrow.

 We’re talking about in some squeezes just a few centimeters away from your chests. For Arasmos was not an easy task to go all the way up to the surface. Today with the modern technology we have good equipment not easy. But years back when Arasmus was walking around the candle. Crazy times. As we can see the exit right here to us going up.

 He was going towards the valley around 15 km from here for the supplies and food. Definitely it was not easy to bring all the supplies through the narrow passage all the way to the castle. The castle was hidden. The exit right here was really hidden. So, this was a best hiding place. The caves of Pajama lie at the start of a major system.

 Mayan has great knowledge of two chambers that lie 9 km further into the system, worn out over millennia by the river Puca. Here in Slovenia, I would say that we have two Slovenia. One Slovenia is on the surface and other Slovenia’s under the ground. Never in my life, I’ll explore all the caves in in [Music] Slovenia.

 Slovenia has more than 13,000 caves, and experts believe there are many more yet to be discovered. At the beginning of cave exploration, there are different stories and legends of devils possessing the caves. In middle ages that people actually thought that cave is the portal door of hell. The black cave is around 3 km in length.

 It was discovered in the 19th century when the ceiling collapsed into a sinkhole. The cave was named after syi deposits which have stained the calsite formations a dark gray when they were once white. The theory is that the burst cave explorers when they entering the cave use lanterns, candles, torches and all this smoke deposit on formations and that’s why we can see the black color.

[Music] The neighboring cave, the Puca cave, is 65 m below the surface and takes its name from the river which created it. It’s 5 km in length. This is known as active cave. The water is flowing, eroding the rock and actually shaping the cave. It’s important to know in which direction the water is flowing.

 So we need to protect the water. And if we know the cave better, then we can protect them this way. The Black Cave and the Puca Cave are the last two on the river before it disappears into inaccessible depths at least 150 m below the surface. The Puca then reappears flowing towards Slovenia’s capital, Ljubljana, 50 km away.

Some of the caves are also show caves. So, it’s good to show people what is underground that people can appreciate nature and they can also protect and respect the nature for uh future generations. Experts like Mayanne are proud to show tourists the beauty of these underground caverns, but free access to some of them has been restricted to ensure the caves are not damaged by visitors.

 The Atoska cave is now only accessible with an official guide. The first visitors when they enter in the cave take some souvenirs out, some rocks and so on. There was some damage being done inside. When Atka was first discovered a century ago, it looked magnificent, packed with hundreds of stallctites just like these in the neighboring cave.

 But in the last 100 years, most of Atka’s stallctites have been snapped off and stolen by souvenir hunters. While graffiti and paint was dorbed on precious formations that had taken millennia to develop, Mayanne is going to show SA the damage. So here we can see some old signatures and the dates they go back 1923.

 So of course this means pollution to the underground, pollution to the cave. Some signatures can be inside caves for many hundreds, even thousands of years. A few names written on a wall may not seem that big a deal, but the beauty of this glittering surface has now been tarnished. It could take many decades for the delicate crystalline formation to grow back over it.

 And for new stallctites to form over these snapped off stumps will take 100 years per cm. Takes time, 2 3,000 years. So it will heal up and become more beautiful cave. Slovenia’s precious cave heritage is worth protecting. The most special thing for me about these caves are these crystals. [Music] Crystallization happens when calsite is left behind in the dry limestone.

You point a light and it’s like it it comes to life with that glittering, with that sparkling. And for me, that is always a very special feeling. It’s almost like it’s not real. It’s so nice to see this cave healing. I think it’s an important lesson that we can preserve cave for generations and generations to come.

The Atoska cave was formed by the Puca River thousands of years ago, but the fast flowing water gradually wore out a new route underneath. To see this river below, one must trek far deeper into Atoska. It’s getting louder and louder. You can see the river flowing. Yeah. I’m close here. Wow. So, you can see the water level here on the line on the wall.

 So, it really can be much higher than it is at the moment. No, it can be 8 9 m higher than is now. The Bucer River races through these underground caverns at speeds of up to 40 km hour, continuing its remarkable work of thousands of years, carving out new underground worlds. It is kind of special to see the the river in the cave and be so close to it.

 Here in Slovenia we have long history of cave exploration going all the way back of time of Azum Praamski who was walking in the praama castle cave. So we need to understand Slovenian underground to simply preserve it. These underground worlds were here long before man ever walked this earth and will still exist once we are gone. For years we have used them for shelter, protection, and survival.

 As we look into the next century, it is our turn to protect these precious subterranean spaces. [Music] [Music] In World War II, why did Hitler convert the Channel Islands into an impregnable underground fortress? If there had been a gas attack, this was a decontamination zone. How did a littleknown ancient civilization use this labyrinth to wage a guerilla war against their powerful and aggressive neighbors? That they can attract enemy soldiers into tunnels and you would be locked in there forever.

And how have two underground worlds transformed the fame and fortune of a city? Nice invention. This brewery started beer revolution. Beneath our feet lie extraordinary spaces, caves and tunnels. The span and the size is just crazy. They’ve been designed and built by us. This is the only one with the castle as well as formed by nature.

 But how were they created and adapted? By who and why? You’ve got to face your fears. Throughout history, subterranean life has captured our imagination. Feel so privileged. We’re going further and deeper to unearth the mysteries, the stories, and the secrets of underground worlds. 30 kilometers off the coast of France lies the archipelago known as the Channel Islands.

 The largest of them is Jersey with a population of around 100,000 people in an area of 120 km. During World War II, the islands were occupied by Germany under the orders of Adolf Hitler. But why did the Furer take such an interest in the Channel Islands? It was British territory. Although Hitler never came here, I think he got a bit obsessed by it.

 What did it take to convert the island into an impenetrable system of underground fortified bunkers? Jersey was the most fortified place in the world. And at what cost? The Nazi party racial views regarded the Slavic races as subhuman. The Channel Islands are not part of the United Kingdom, but since they are dependent territories of the crown, the UK is responsible for their defense.

 From September 1939 to spring 1940, Jersey and the other Channel Islands were largely untouched by the war. But by June 1940, following the German defeat of the Allies in France and their evacuation from Dunkirk, the British government decreed that the Channel Islands had no strategic value. They would not be defended and instead were left open to the Germans.

 Mark Yates, a battlefield guide, is an expert on Jerseyy’s military history. He’s entering an incredible underground system that would be created by an invading German force in the vacuum left by the departing British. What little British military resources were here were taken off and then there was a general evacuation of the population of the island.

In the 1940s, the population of Jersey stood at 50,000. Nearly 7,000 residents decided to leave, leaving 43,000 behind to face the German occupation. It was British territory. Although Hitler never came here, um you he got a bit obsessed by it. And so at that point, they organized Operation Green Arrow.

 The purpose of the the operation was to occupy the Channel Islands as part of the occupation of France. The German air force, the Luftvafa, landed on Jersey on June 30th, 1940, and the island’s ruling committee surrendered the very next day. The arrival of the Germans and being under occupation must have been pretty scary at the outset.

 Um, and the unknown must have been really quite worrying, particularly for families where you had young children. Trevor Green, now 82, was just a boy when the Germans marched into town in their thousands. when I was about five that I first came into contact with the Germans where I lived at a place called Milard’s Corner and on the coast road and so they were the ones that really came to my attention.

Hitler expected that the allies would one day attack his Nazi occupied territories in Europe. He obsessed over the creation of a coastal defense system, which he called the Atlantic Wall. It was a series of fortresses, gun imp placements, tank traps, and other obstacles running for more than 3,000 km along the coast.

 The Germans turned Jersey into a fortress of bunker systems housing radar and observation units. They also planned nine ocean-facing artillery posts like this one at Battery Mula in the northwest of the island. The gun imp placements sit above a network of bomb-proof tunnels. Trevor Green has come to meet Tony Pike, an expert on Jerseyy’s military history to discover what the German invaders built on this island.

This is actually constructed by making a deep trench which would been about 4 m deep in excavation. And once they’ve actually done that excavation, they would have poured the the floor. Then they would have made the shuttering for the walls, which you could still see the imprint today.

 And once they’ve done all that wooden shuttering, they then poured the concrete behind. And then they would have done the shuttering for the roof, poured the concrete on top of that. And then all the infill which would have been used excavated from the trench itself would been put on top of that. The structure at Battery Mula was built from concrete and steel, most of which needed to be shipped in from France.

 The battery included underground bunkers, a personal shelter for 27 men, and four ammunition stores, which served the gun placement situated above ground. Excavating this vast network wasn’t an easy task. talking about solid granite and to to mine it to actually you know create an excavation 4 m deep and more it took a lot of work quite a lot of blasting as well with gelignite but they also used a lot of manual labor the 43,000 residents of Jersey who stayed behind after the evacuation refused to do the Germans dirty work the channel islands being proudly British working

for the Germans at the beginning of the occupation was really something that nobody really wanted to do. Battery Mula and other German installations on the island were constructed by a workforce of mostly prisoners and conscripts who were treated as slaves. They were shipped in from Nazi occupied territories all over the continent.

 They came from the eastern front from Eastern Europe. They were typically uh Soviet civilians who were conscripted or Soviet prisoners of war. There were hundreds of thousands of these these people that the Germans used as labor across Europe. And there’s a disproportionate number of Russian laborers who were injured and killed on the construction sites around the Channel Islands.

 Locked up in camps across the Channel Islands, a workforce of 16,000 were treated brutally and forced to work in dangerous conditions. The Germans reserved the harshest treatment of all for central, southern, and eastern Europeans who spoke Slavic languages. The Nazi party racial views regarded the Slavic races as subhuman. They were brutally treated.

And of course, in an environment where you’re starved, you’re not in the best of health, and you’re working on construction sites where, to be honest, there’s no health and safety, who’s most likely to suffer? On Jersey, over 6,000 slave laborers worked more than 12-hour days.

 Under constant fear of rockfalls, they excavated 1,000 m of tunnels using just simple picks and shovels. the force workers and the slave workers actually built this site. Um that’s very important to remember what they did. You know, is a colossal amount of work they did in all sorts of different weathers um with very poor equipment.

 A lot of the time slave worker, what they had for protective clothing was an old cement sack with a couple of arms just punched through. Due to cement shortages in summer 1943, the Germans abandoned their original plan to build eight batteries on Jersey. Only four army and two marine batteries were completed, including battery Mula, which contains a gun imp placement, ammo stores, and a gas lock chamber, all connected by 4 m deep tunnels.

 This personnel shelter is actually made to be contrary against gas attack because Hitler was gassed in the First World War. He didn’t want his troops to go through the same experience. So, a lot of engineering and thought has actually gone into this bunker. This part here, we’re standing underneath a shower head. So, if there had been a gas attack, this was a decontamination zone.

 So, the idea was you had a shower here. The water was heated from the boiler next door and then you would go into this part which is a gas lock. The door through there would have been already closed. Then we walk straight through here into the bunker. In the event of a mustard gas attack from the Allies, these steel doors would be locked and sealed while pumps would filter out poisonous gases.

 This part of the bunker, the air would be cleansed and filtered. When a green light shone that was placed up there shone, then it was safe to open this door and enter into the main part of the complex. The whole the the whole engineering in this bunker is absolutely magnificent. At the heart of the bunker are the filtration pumps designed to keep the men safe from lethal gases.

 We’ve had to re restore this bunker. This was a complete wreck. This bunker when we first started back in 1979 and we’ve rigged it up so it runs today with a modern motor, but it also could be cranked round. So, I’ll show you now how it actually functions. So, that would take in the air. Well, taking in the air from the outside here. Air from above the cliffs would be pumped in through a network of pipes while carbon filters cleaned the air making it safe to breathe.

 I see it’s got a handle. What is that for? Well, if there had been an electrical failure, although battery malta had its own generation plant, but should say for example, if there had been a bomber hit that and it had been disabled for some reason or another and there had been a gas attack, it’s always a backup plan. You could actually crank this round by hand.

Because Hitler decreed that 10% of the steel and concrete used in the entire Atlantic wall should go to the Channel Islands. They ended up as the most heavily fortified region along the Nazi coastal frontier. That equates to 484,000 cubic meters of concrete. You know, that’s a hell of a lot of concrete just in three small arms. Yeah. Oh, yes.

Well, when you work it out, I do know that from a size point of view with the fortifications that they did that Jersey was the most fortified place in the world. Tony Pike has been involved in digging out and restoring Jerseyy’s wartime tunnel heritage since 1979 when he was a teenager.

 He worked on the tunnels under Battery Mula. When I was a 15year-old lad, I came down here for the first time and we came through one of the entrances which we had to dig out by hand. This is one of the ready entrances for the the crew that would have gone up to man the gun. And at the time the rubble was virtually up to the here up to the roof.

 We come down and we literally passed out all this rubble by hand up into a skip above. As well as the coastal batteries, the Germans built numerous tunnels and bunkers across the Channel Islands, even hospital accommodation for their expected casualties in the event of counter invasion. Work is still underway to dig them all out and open them to the public.

 This naval battery known as type FL242 anti-aircraft bunker was excavated in 2016. Tony and the team were surprised to find artwork left there by German personnel. When I first came in here, I just literally squeezed in through the entrance. The rubble and dirt was literally only 2 foot off the ceiling and the first thing I saw was the flowers. Yeah, it’s amazing.

 And then I came through into the gas lock, which would have been the gas lock, and we found even more of these flowers. And when you look at the detail, it is quite amazing. They must have really wanted it to make make it feel like home. When the war ended in 1945, the liberated island folk wanted their home cleansed of the taint of German occupation.

 The British army was given the complicated task. More than 60,000 mines and 30,000 tons of ammunition had to be removed while the guns of Battery Mula were thrown over the cliffs. Over the decades that followed, the abandoned bunkers were used as dumping grounds. But with the help of volunteers like Tony, the doors of history are at last being pushed open to offer everyone a glimpse into Jerseyy’s past.

 When you think this was worked on by the forced labor and the slaves to to build it and to achieve this, this was a lot of hard work. This bunker complex is quite incredible. [Music] With a land mass of 780,000 square kilm, Turkey is a substantial country bridging Europe and the Middle East. Its population of 80 million now lives in relative peace.

 But 3,000 years ago, the country was a battleground for waring civilizations. The central region of Capidokia is famous for its pockmarked landscape of volcanic rock called Tufa which is formed by compressed volcanic ash and for a series of extraordinary underground worlds dating back to the 8th century BC.

 It’s amazing to see what people are capable of for survival. How did craftsmen engineer these spaces for secure living underground? only one to one can fight in those sort of narrow tunnels so that the big army lose their advantage. And how have these tunnels been adapted over the centuries to give them a purpose in the modern era? It’s not only beautiful, it’s ecological.

 Kimacla is 310 km southeast of Turkeykey’s capital, Anchora. The sprawling hilltown conceals numerous entrances to a hidden world. This is one of at least 20 underground cities in the Capidokia region. Built on eight levels, 40 m deep, Kimacla has space for 5,000 people to live comfortably for months at a time. [Music] Husam Sulimangal, a long-erving guide, knows the tunnels intimately.

 Now, I came here for the first time about 35 years ago. It’s a maze of tunnels and rooms and different functions, and I was not imagined it would be this big. Now only four levels of the Kimacla complex are open to the public and Husam’s passion to discover the story of the city’s origins has kept him coming back year after year.

 None of those underground cities in the area of Capedokia, they couldn’t find anything written. They couldn’t find anything organic. They couldn’t find any tools or anything. So it’s a mysterious thing. You can’t tell when it was made and by whom it was made. Despite the uncertainty, historians have estimated kimacula dates from 800 years BC.

 So at a time when the tribes of Western Europe were learning how to make their first Bronze Age tools, it’s thought the people here, the Fraians, were developing a network of underground fortresses to entrap their enemies, the Assyrians. The theory that is most plausible to me, it’s done by Friians around 8th century BC for military reasons to stop an Assyrian army.

 uh by waging a guerilla type of warfare. [Music] The Assyrians were a powerful people whose empire spanned the Middle East and lasted for almost 2,000 years, ending around the 6th century BC. The Fraians of central Turkey built their underground cities like Kimacla to both hide from and retaliate against the Assyrians.

basically attack the major army which is the Assyrians and as the army gathers up and chase them they would just run back hide into those small underground cities. So the entrances and tunnels are especially made narrow and low in sailing for the defensive purposes so that the big army can lose their advantage of being being a crowded.

 So it’s only one to one can can fight in those sort of narrow tunnels and doesn’t matter if you have thousand people behind it or 10 they became sort of equal. The Fraians had another clever tactic in mind. After making their enemies vulnerable in this confined space they had a network of traps ready to lock them in.

There are different sections of this whole underground city that are connected to each other by narrow tunnels where they can block it with millstone looking rolling doors. If you don’t know where you’re going, you can easily get lost and fall into traps. Of course, imagine that it’s all dark in those days.

 You have to carry your own torches. So, if the enemy would come in, they can just empty one area, go hide in the other part, and block themselves in. But if you’re the familiar one that you can always find your way out from a different secret entrance. Circular doors carved from rock were designed to roll into place and block the tunnel entrances.

 And here’s one of those rolling doors where they would attract enemy soldiers into tunnels and then trap them in. Roll the door, put a peg behind it, and you would be locked in there forever. Having trapped their enemies in the tunnels, the Fragians had another trick to defeat them, Kimacla is secretly linked to other underground cities in Capidokia.

 Then they also have escape routes. Then they would just run out. And some of those underground cities are connected to each other with um about 67 miles long tunnels. So they can come in into one of those underground cities and escape from the other one and still attack the army. It appears the Fragians built Kimacla as a refuge.

 In later centuries, the many underground spaces across the region have found new uses. Today, architect Ala Usuzbai lives and works in a cave house. I feel very safe. I think it has some kind of um protection feeling in a cave house which has a natural sunlight window. It’s not only safe, it’s beautiful. It’s it’s like you are living in a sculpture.

 It’s not only beautiful, it’s ecological. Ala has been breathing new life into the area’s disused caves. It’s a privilege to be able to work here as an architect because Kapodoki is a very exceptional unique place. There are no straight lines, no rational decisions made by an architect or a constructor. Uh the whole thing is done uh by whoever has carved it, the mason.

 It’s very unique and it’s very exciting. You don’t get bored. Ala’s perspective on the changing uses of the Capidokian caves may throw some light on what’s happened in Kimacla through the centuries since the Fragians occupied the underground city. These spaces look like archaeological sites, but actually they’re not. They have been constant living spaces for about 3,000 years.

 Kaaklu is actually about a 8story huge uh underground living system. So it’s amazing to see what people are capable of for survival. Historians have found evidence of continuous use of kyala from ancient civilizations right through to the present. About 2,000 years ago, Hittites have carved a space for something. Then Romans came, then came the Byzantine period, then came the Turks.

 And you discover how they used to live inside spaces. In time, if it’s flexible enough, people can uh change that space for their contemporary needs from their different perspectives. Architect Ala and historian Hassan can understand how the civilizations who occupied Kimacla found different uses for it.

 How would they build this place? How did they manage to calculate where they need a pillar to support the lower or upper floors? And how do they know without using compasses or GPSs as we now have? And even though it’s very soft, it’s a very sound structure. They have about 2 m or 2 and 1/2 mters thick layers in between the floors.

 So this is not like the uh beams that we know of. And this is the largest opening that they make. It’s very ecological actually, you know. It’s it’s very easy to keep the heat inside. Oh yeah. So It’s insulating. It’s insulating. And I don’t think uh they actually designed it the way we do it today. They they lived through uh experience. It’s not planned.

 It’s not planned. So they uh it should have taken hundreds of years. I don’t know how long. And no one knows actually. No one knows. Yes. That’s the mystery of it. There are clues in the design of each underground space which suggest that they had specific uses. For instance, in one chamber on the fourth level, 27 m below the entrance, Ham has an explanation for this unusual looking vessel carved into a small pit.

 Now, here we are. You have uh the wine press where imagine in those days mostly women would be going in pressing the grapes with their feet. Extracted juice would be coming out from this hole into that container because the temperature is pretty fixed here. It’s a damn good place for fermenting the wine and keep aging them.

At the same depth, there are signs of organized kitchens and the preservation and storage of large quantities of food because of the volumes of stoages. You can tell two or 300 people living in the system. So it’s amazing uh trying to understand how they could manage. The rock walls suggest clues about the ingenious techniques they would have used in food preservation.

 They stick branches into holes and they used to hang their um fruits and vegetables onto the branches. So that’s how they dried them out. The kitchen area includes other tools such as a stone grain mill. put a small wooden handle here, gradually turning it and grinding the grains, obtaining flour to make their breads or whatever else.

Other details in the rock suggest that livestock was kept here. When you see an animal tied hole here, it shouldn’t be at the same spot where they dry their uh fruits and tie their animals. So you understand that probably uh before it was a storage then in time it became a barn kind of. By the 4th century AD the Capidokia region was part of the Baantine Empire a powerful cultural and military force rooted in Orthodox Christianity.

 This room is known as the church. The local bishop St. Basil the great founded numerous places of worship in the region and Kylo boasts a small Orthodox church. Monastic orders started here by the guy called Basilius the Great around 4th century AD. They’re trying to imitate a normal church construction.

 There is one small niche here where you can see it’s also blackened. Uh there would they would put a small terra cotta oil lamp in here. Another one is right there. example that arch separating the holiest part of the church from the rest of the congregation because in the Greek Orthodox Church Eucharist the sharing of wine and bread is happening only among two priests and that needs to be behind this wall.

The last Christians left in 1923 when the modern state of Turkey was founded. Abandoned for 40 years, they were finally restored and open to the public in 1964. took us quite a long time in those days trying to figure out what is what. Now they’ve got nice signs. But the old Capidokian traditions of underground dwelling are enjoying a resurgence thanks to the work of architects like Ala turning old caves into remarkable new living spaces.

 This is modern Capidopia. [Music] One has been turned into a bathroom space. You can still see the original niches with a fireplace in the middle. Here we are in the old stable room and today uh we are using it as a nice wardrobe room. The underground city of Kimacula is a fascinating puzzle for historians who continue to look for clues about what happened here in centuries past.

 What is certain is that for thousands of years, people lived, ate, prayed, and kept animals here. What’s more, they designed these tunnels to trap and outwit their enemies. So the Capidokian tradition is a testament to human [Music] ingenuity. The Czech Republic is a country in central Europe with a population of just over 10 million. Pilson is the fourth largest city 90 km west of the Czech capital Prague in an ancient kingdom known as Bohemia.

 There is a fine history of brewing here dating back to the 14th century. A tradition which has given rise to an extraordinary network of cellars, wells and tunnels which lie under the city. In every century after people expanded the underground a little bit more. But how did these secret passages give birth to the most popular type of beer ever? The water which filtered through the sandstone, it was very pure.

 Where did they harness the power of nature to create this liquid gold? So we had more than 60 of these rooms down here. And what effect did Pilson’s brewery have on the rest of the world? This brewery started a beer revolution. Pilson was founded in 1295 by a Bohemian king Weslus II on an important trade route at the meeting point of four rivers.

 The city is remarkable because it is the location of not one but two extraordinary underground worlds. one dating back to the 13th century and the other to the 19th. Why did they excavate these tunnels and vaulted chambers in the first place? Local historian Yan Hus was first brought here aged 14 by his father. Ever since he has been fascinated by the development of this underground world.

I expect couple of rooms, couple of corridors, but uh when I saw how big it actually is, like a 13 kilometers and two floors, I was kind of surprised. Beneath the historic city of Pilson lies a network of cellers, wells and store rooms linked together by a complex chain of interconnecting tunnels. The whole system of prisons underground was uh created between 13th and 18th century.

As he makes his way into this labyrinth, Yan is traveling back through time to the beginning of the Pilson story. Local monks brewed beer here in the 1300s. The water supply was ideal because of its low alkalinity. Towns people wanted to make their own beer and sunk wells to get at the water.

 Here we can see the really narrow part which actually leads to medieval well. So people created many wells to get access to the underground water because 300 houses in medieval pen made a beer during medieval times. So each one of these houses had wells. Pilson’s towns folk were not obsessed with drinking beer for pleasure.

 Europe was rife with plague and waterbornne diseases during the Middle Ages which meant that it was safer for people to drink beer rather than water for hydration because the fermentation process kills unwanted bacteria. Here one medieval well that people created. When you look up you can see how much underground we actually are.

 We’re about seven maybe 8 m under the street. The people of Pilsson dug wells deep into the soft sandstone. In order to extract the water, they used techniques that were ingenious for the era. In the medieval Pilzen or in the city center, there used to be 360 wells. So even during the 15th century, people didn’t use some buckets and ropes to get the water from these wells and they pumped the water out using pressure.

 And these pipes are wooden pipes from 15th century. To make these pipes, which are remarkably still intact after 600 years, it was important to pick a variety of tree that was both straight and sturdy. People used fur trees because it was easier to drill through than, for example, the oak. They used a hot piece of iron and they like burned their way through.

 Then people placed them inside a well with a pump on the surface. To protect and waterproof the wood as best they could, they took advantage of a preservation technique that had been used since ancient times. They painted them originally, but not with the normal paint or resin. They choose blood, ox blood.

 The albamin protein in the blood made the perfect water resistant coating. Having perfected their digging and plumbing methods, the 14th century residents of Pilson carved out sellers for storage and brewing. Many homes in the city have sellers running three stories deep over the next 400 years. They went on to build a network of tunnels to link the sellers together.

The labyrinth eventually extended to 13 km. It was completely dark in here. No lights, no torches whatsoever. So each person walking down here carried their own like a candlestick or oil lamp as a source of light here where we can enter one of the first medieval corridors. The oldest corridors here are only a meter and a half tall.

 Today an adult of average height would struggle to fit through them. People used to walk through this without a problem. During 13th century those people were really tiny. Down it’s like more narrow for legs and up it’s kind of wider for shoulders. It is not comfortable but they fitted just right.

 The labyrinth grew organically as individual households carved out their basements to suit their own needs. During medieval times, each house had some sort of access down here. This is a taller space definitely because uh the underground during medieval times was created by different people uh in the city and different people had a different opinions on how it’s supposed to look.

 So some curts are really like a narrow short some are big open spaces like this. Sandstone is a permeable sedimentary rock which can be unstable. As the labyrinth expanded and the width of the corridors increased, it became necessary to support them to stop them from caving in. The whole complex was created only by some hand tools.

 It’s actually really soft rocks, but also there used to be a lot of underground water. So the sandstone, it started to crack on many places. So people had to reinforce it somehow. So they added like the brick arches or brick ceilings to like keep the weight up. So they started expanding the quarters more and more and every century after people expanded the underground a little bit more and it went on and all until the 18th century when the construction was finished.

Four centuries worth of tunneling knowledge gave the people of Pilson the upper hand in times of war. Pilson’s underground is also the reason why city was well defended during medieval times because it was used as a part of the city’s defense. Pilson’s residents were able to evade their enemies and use the tunnels to spring stealth attacks on invaders.

 People from Pilson were hiding down here during war and for example some fires in the city and also organizing some attacks on the attackers themselves. So it was like the hidden run tactic and thanks to that actually Ben was quite successful in its defense. It was conquered basically only once in the 17th century in the year 1618. As well as its defense benefit, the residents made full use of the labyrinth’s cool ambient temperature.

This was integral to the development of the brewing industry for which Pilson was to become world famous. Originally, people in medieval pen used the underground mostly as a storage. storage for their food, water, but mostly the beer barrels because there’s a colder temperature down here than in the in the city.

 People used it as a giant fridge. Beer fermented down here also aged or matured in the closed barrels in these like a arches. The history of Pilson and its brewing tradition are intimately entwined with this city’s subterranean character. You can literally touch the history and some of these these parts from like a 13th century to present day.

 By the 19th century, Pilson’s cottage industry of brewing in private sellers was about to be revolutionized. Beer quality varied from one small brewery to another. So the city fathers founded a largecale brewery to standardize their product. The PSNA Urkl Brewery opened in 1839. Here a unique form of beer was developed which was to become one of the world’s most popular drinks.

 We are here in the sellers where was born prisoner beer. Vaklav Burka has held a key role in the brewery since 1982. He’s the brew master like his father before him. Brew master is uh I would say most important person in the brewery. He’s responsible for the proper uh brewing and fermentation process. Verlav is preserving a tradition that connects back to the original invention of pills beer.

 We still have recipe uh and process which was set in year 1842 mean more than one 176 years ago by Joseph Gro. It’s unbelievable. Joseph Gro was the first brew master in Pilson. He came from Bavaria which is now part of Germany. Gro was experimenting with laga, a new beer making process spreading across Europe in the mid-9th century.

 By adapting the technique to suit the local soft water, Gro ended up creating the first golden laga which was called Pznner after the town. The water which is coming to Pilzen filtered through the sandstone uh very pure uh water and very soft water. For brewing we still use only three ingredients like Joseph Gro in 1842.

 Malt, barley and hops are combined with water to make laga. The name comes from the German for storehouse because the beer matures slowly and at very low temperatures stored in caves or cellers. When we started production here in this brewery uh was very important that we needed to dug new sellers. So in the 1840s when the people of Pilson needed to brew laga on an industrial scale, they needed enormous cooling cellers to ferment their beer.

 A city that already had one massive labyrinth was about to dig itself another one. Historian Yan has come to look at the staggering number of rooms built specially to create a rudimentary refrigeration system. So this was the ice storage room or a fridge. So we had more than 60 of these rooms down here in the cellar because our beer it must ferment and also age during low temperatures usually 5 or 7° of Celsius but the natural temperature in the cellar is higher.

 Every year at the beginning of winter people from the brewery they traveled to ponds and lakes. They gathered blocks of ice over there and the ice was brought down here into the rooms just like this one. People used the shaft up there to throw it down here. The innovation in these enormous sellers was that the ice would slowly melt and trickle into a series of channels running along the floor to cool every room throughout the brewery.

 The ice water was flowing through all these sellers and cool this granite stone on the floor. And it was another possibility how we cool the sellers in past before invention of artificial cooling. Nice invention. The scale and innovation continues in these giant cellers where fermentation and maturation took place. Each cellar is 7 m wide and 9 m deep.

There used to be beer barrels everywhere down here. every stop in every room, every corridor. Also, there was a like a small railway system down here, which was like the loading system for the the barrels and our biggest barrel that we have in this quarter. It can hold almost 10,000 L of beer.

 This underground world is a wonder of 19th century creativity, much copied, but rarely bettered. The beer that we make, our lagger, became basically the first pale lagger in the world. This beer inspired many other breweries. Pilson’s worldrenowned drink owes its popularity to a fine tradition of underground excavation.

 Without this underground will be not so popular style PSNA beer. It’s remarkable to think that a system of wells, cellers, and tunnels dating back to the 13th century led to the creation of a brewery in the 19th century, which in turn relied on a brilliant new system of tunnels and ice water channels to produce its prime laga product. Now, a global success story.

This brewery started, I would say, uh, beer revolution. [Music] [Music] How does the world’s most peaceful nation explain its obsession with nuclear bunkers? This is the Swiss concept. We have chocolate, the Alps, and we have our bunkers. Digging beneath a British city, what brought these volunteers to a state of ecstasy? We found this arch from above. was screams of joy. We’re in.

 We found it. We found it. And how did Slovenia’s young adventurers strike gold in a disused lead mine? This is the only underground kayaking route in the world. It’s pretty special to come here. You never know what to expect. Beneath our feet lie extraordinary spaces, caves, and tunnels. The span and the size is just crazy.

 They’ve been designed and built by us. This is the only one with the castle as well as formed by nature. But how were they created and adapted? By who and why? You’ve got to face your fears. Throughout history, subterranean life has captured our imagination. Feel so privileged. We’re going further and deeper to unearth the mysteries, the stories, and the secrets of underground worlds.

 Switzerland, a peaceloving nation at the heart of Europe. All its neighbors, Germany, France, and Italy, have been dragged into military conflicts over the last century. While Switzerland always stayed neutral, never engaging in war. The country has also fiercely defended its independence. Its government has a master plan to protect every citizen in the event of a nuclear war, which involves going underground.

Even if you’re neutral to a war, you still need some protection. Swiss law decrees that every citizen must have a place in a fallout shelter. These buildings just demonstrate how massive the threat perception must have been. There are now 300,000 of these shelters in Switzerland. Ample room for the entire population.

 Underground guard, you’re safe. But it’s not an easy life. Photographer Ddier Ruff has spent more than 30 years documenting Switzerland’s underground worlds. In Switzerland, the bunkers are everywhere. In the private house, in the schools, in factories, in hospitals, everywhere. Throughout the 20th century, Switzerland feared military aggression.

 During the Cold War, the threat was nuclear attack. child of the 60s, the threat was a home attack and in that case it was needed to have the shelter. This uh protected on the world bunker and this is where it all started. However, the Swiss were digging underground way before the nuclear threat. In 1939, just before World War II, Switzerland strengthened its border defenses along the river Rine.

 The threat of German invasion here was all too real. So the Swiss built the mighty Fesong Fortress. This vast fortified bunker was the first line of defense and is sunk 20 m into the hillside. As part of his research, Ddier has come to the fortress to meet historian Walter Louie. We are right there on the border about 500 m further around you.

 You will come to the riverine and the other side is Germany. Germany. Yes. Walter has been caretaker for the last 20 years and is an expert on its military history. Well, if originally planned and built for for 90 men, then we had decreased up to 160. Now we are entering the main galley. We are here about 15 m underground and it’s getting deeper and steeper.

The main tunnel is 210 m long and provides access to the four observation towers. Two of them armed with 75 mm long range rapid firing guns. So it closes automatically. He so the empty shell out. Next one goes in and so on. These semi-automatic guns had a range of 11 km and were capable of firing up to 20 shots a minute.

 Soldiers would train them on their targets using numeric charts that were advanced for the era. You look through there and you see points. Each one has a number and he turns the wheel till he has it and then he’s ready. That’s for the time. One of the most modern guns which exist, Feson was ready by 1939 and used to monitor the German border throughout the Second World War.

 The Swiss army only decommissioned the fortress in 1988. At the height of the war, up to 100 soldiers would be stationed underground here for up to 1 month in a state of constant alert. Their living quarters included basic wooden bunk beds, a large kitchen, and a hospital with a surgery. Underground, you are safe.

 But it is depressing if you stay for days and days and you never come out of it. You you are losing the sense for for day and night. It’s not an easy life. must been terrible to be to be here locked inside like mouses with no sun, no no terrible. At the center of the fortress are the vital amenities needed to keep the men alive during each month-long vigil.

 There are two diesel generators, one acting as a backup along with water tanks and air purifiers. had two sets of filter, one running, one standby and then the air goes into this distribution and throughout the floor it’s it’s distributed. The air supply had to be heated to 30° to stop the gunner’s glasses and gas masks from steaming up.

 Water we had 60,000 liters that would have lasted for about uh 6 weeks. food. Well, we had about 3 weeks, but air that that’s crucial. That’s crucial. The deepest tunnel ran 20 m into the mountain and offered soldiers an escape route if needed. Soft. And that leads us to a banko. It’s also used as emergency exit. You want to go? There’s a lot of noise.

 That is the engine. This deepest tunnel is 290 m long. To me, this place is like a submarine. Is a is a place where you lose contact with reality, with time, and uh you’re totally uh closed in a cellar. I’m I’m a bit lost in this underground alley now. I don’t know exactly where we are anymore. It’s confusing. You can’t get easily.

You can’t get lost. I see this place as a sign of the past. Swiss has always been fighting to protect this small territory and the bunker would be the way to protect the population. Thanks to military defenses like the Fesong Fortress, Switzerland was not invaded during World War II. The Nazis resorted to a war of words, calling Switzerland a medieval remnant.

 But the Fesong installation proves how modern and efficient the Swiss capabilities were. When the war was over, the threat of Nazi invasion was replaced by fear of nuclear attack during the Cold War arms race. Historian Dr. Michael Olsenski explains how the Swiss love affair with underground bunkers grew more passionate as a result.

 The Swiss authorities uh finalized the plan that every Swiss citizen should have his own safe place in the case of war. Every community in Switzerland has to have this kind of protecting installations. At the height of the Cold War in the 1960s, plans to improve the motorway network around the city of Lousern paved the way for an incredible engineering project directly underneath a normallooking road [Music] tunnel.

 They were planning to build two tunnels. A clever politician came up with the idea that they could design the tunnels in such a way so that they could take a secondary bunker function. Over 10 years, Zora Shelbear has got to know every inch of this place, the Sonnenberg bunker. Slowly, Zora is going to show Dier how a massive public bunker like Sonnenberg is organized.

In here, you get a good idea of the whole height of the building, which is 20 m. 20 m high. Yes. In the buildup to an attack, the tunnel doors would close. 700 staff would transform this underground facility into a superersized bunker for 20,000 people. It’s the ultimate example of Switzerland’s commitment to protecting its citizens.

 As with the World War II fortress, air supply is vital. But while Fesong had two filtration units, the Sonnenberg bunker has 120. All of these filters here were designed to filter any then known nuclear, biological, and chemical agent so that the 20,000 people in the two tunnels could have been provided with fresh air if everything had been contaminated outside.

This reminds me a lot about the filtration system that that we have in the private bunkers. So, it’s just the scale is just massive. Yes. Because it was designed uh for a third of our city’s inhabitants. Those 20,000 citizens would have been divided up into groups of around 750 people occupying different zones underneath the motorway tunnel and inside the central sevenstory facility that’s hidden underground.

 450 tons of flatpack furniture still lies in weight for them. So this room you would have been sharing with another 63 people. Imagine 30, 40 people tossing, turning, snoring, coughing, crying. Unimaginable. The sanitation facilities are basic with no showers and would have been shared by hundreds of people. Water would have been taken from the communal water supply, from purified groundwater and the city river.

And if these three options had failed because of contamination, they would have had water tanks. Water tanks. Okay. But it would have been rationed to 4 L per person a day. They thought half of it for hydration and the other half for hygienic purposes. And after 2 weeks, well 2 weeks is just as far as the plan went. Went. Yes.

The logistics center is in the heart of the structure. It contains everything needed to run this enormous underground bunker. This is the top level which used to be the security station with prison cells for potential tunnel troublemakers. Provision was made in the prison wing to keep only 16 people locked up out of 20,000 inhabitants.

And the color, by the way, is still the original paint. You’ll see a lot of yellow and green, a little bit of color psychology, yellow for sunlight, green for nature. The command post was on the upper level. The civil defense team would run the facility from here. They had telecommunications facilities and a radio station to broadcast to other bunkers around the country.

 This would have been their sole means of communication with the outside world. Every now and then they would have played some music as well. Probably something soothing to not upset people. [Music] [Applause] [Music] Obviously also they would have chosen any news very wisely. Of course, things that would keep their hopes up.

 M uh hidden away in the facility is an underground hospital spread over three floors with two operating theaters. This room in here is the only room that is not painted in green or yellow but in this salmon orange salmon. Do you have any idea why? Who? Why this color? For whom? No idea. was the nursery. Nursery. Apparently, it’s a soothing color.

Nursery. Yeah. Because even during two weeks, there would have been some babies were born. Women Yeah. giving birth. As an architecture, it is amazing. We could almost said it’s an underground city. You have a hospital. You have the bed accommodation. Police expect also a maternity.

 People could be jailed, could give birth and uh life would go on but only 14 days. Yeah. From the logistics center, there is only one way of accessing the tunnels themselves. So the noise is this is traffic in the west tube. Behind this tunnel is the other tunnel and the emergency exit links the two tunnels. Yes. The A2 motorway that runs through the tunnels carries 55,000 vehicles a day.

The concrete blast doors at both ends. They’re 1.5 m thick and 350 tons each and apparently able to withstand the blast of a 1 megaton nuclear bomb as close as 1 kilometer away. When the Cold War came to an end in the early 1990s, the costs of maintaining an underground nuclear shelter were too high. The bunker’s role as a shelter for 20,000 people was finally abandoned in 2006, but the ever cautious Swiss have maintain the central areas with a capacity for 2,000 people just in case. It’s part of the zeitgeist of the

era. These buildings just demonstrate how massive the threat perception must have been in the 60s. is this nice idea utopia that we could protect the people by putting them in a shelter. It was unique and it was saying something deep about Switzerland and its culture and its history. The Swiss are not building mega bunkers anymore, but since the 1960s, their focus has been on providing mini bunkers for all citizens that are considered to be like any other room in your house.

 There are now more than 300,000 of them under residential development and public buildings like hospitals. Diddier has come to Andelfingan north of Zurich to see one of these shared residential shelters. Growing up in Switzerland in every uh building we have these uh personal shelters. He’s meeting an engineer who is tasked with building these private bunkers.

Within the last 15 years, we built around 6,000 large bunkers up to 500 people. We still also built the the smaller shelters around 30,000 I would say. We built this public shelter in 2006. You will see a lot of others similar to this one. 36 people can go inside of this shelter. And they’re supposed to stay how long inside? For at least 14 days with the necessary food and water. 14 days. Yeah.

The room is just 6x 10 m and entry is via a concrete blast door weighing 900 kg. That’s basically the shelter how it looks like. A dry toilet. No water inside because this would be a weakness for the shelter itself. So, it’s not a lot of intimacy. Yeah. Once you’re inside, you are mixed with all your neighbors and family, friends, and and you have to share this uh tiny space.

Yeah, that’s true. There is also a second exit. It’s just for the worst case if you cannot reopen the the blast door that’s always built in every shelter. There is no other country where bunkers are so commonplace. Even if you’re neutral to a war, you still need some protection and still common to build these things.

 And it’s, I would say, necessary because you never know what happens. What you need is a place to rest, a place to breathe, and a place to to survive. It’s a survival kit. This is the Swiss concept in the modern age. It’s part of the tradition. We have chocolate, we have banks, we have uh the Alps, and we have our bunkers.

For more than 80 years, Switzerland has either built or adapted subterranean spaces to protect the safety of its population. What’s special about the Swiss underground world, be it military or for the protection of the civil population? It’s the only country in the world that has a safe place concept like this.

 The threats may have changed over time, but the demand for protection remains. Now with access to more underground bunkers than ever before, the Swiss are ready for every eventuality. [Music] Liverpool, the historic and powerful trading port in the northwest of England. Iconic buildings line the waterfront of a city steeped in maritime history.

 In the east of the city, beneath the streets of Edge Hill, lies an underground world that’s shrouded in mystery. Like the span and the size is just crazy. But how did it get here? Finally reached the bottom and found ourselves 60 ft below ground. And who built it? He was so secretive, borrowing away for 35 years. A group of determined volunteers are slowly revealing this extraordinary creation in a search for answers.

 Keeps us going. It’s a good hobby. Mad, aren’t we? [Music] [Music] For 20 years, volunteer coordinator Chris Isles has been fascinated with the labyrinth of tunnels built by 19th century businessman Joseph Williamson. Well, I’ve always been interested in underground spaces, man-made especially, and I’ve always been aware of the local legend of Joseph Williamson and his tunnels at Edge Hill.

 Joseph Williamson was a tobacco merchant and landowner during the Georgian era. He moved to Liverpool in 1805. People called him the king of Edge Hill or even the mole of Edge Hill. And I remember being intrigued by it and wanting to know more. Joseph Williamson was uh in the tobacco industry. He married the boss’s daughter, made a lot of money.

 He basically inherited the company and became a very rich man. David Bridson of the Heritage Center has researched the full extent of Joseph Williamson’s wealth. So, he was able to retire at the age of 49 with something like £400,000 in the bank. Now, we’ve seen various estimates of how much that would be worth today, but anything from 25 up to 50 million pounds.

 And he then spent the rest of his life paying men using that money to build his tunnels and build the houses on top. In 1805, Joseph Williamson bought up an area of land that was uninhabitable because it was an abandoned quarry. The land around here had been quarried out for sandstone as Liverpool grew and expanded.

 We’ll never know what gave Williamson the idea, but he hit upon this seemingly crazy idea of roofing them over. In effect, Williamson reclaimed abandoned land. He had pillars and tunnels built into the quarry, lined them with bricks, and built streets and houses over the top. But no map was ever left to show what was now sealed underground.

 So we have these beautiful brick and sandstone vaults, all sorts of sizes from a few inches across up to this one with 25 ft. All purely to reclaim the quarry workings and create usable land in the Edge Hill area. Nothing remains of the properties Williamson built apart from the facade of his great house. After he died in 1840, the caverns grew derelictked and were filled in with earth and building waste.

 We knew of the existence of of the chambers on beneath the buildings that were above. In 1995, the tunnels were rediscovered by a group of amateurs, one of whom was Chris Isles. We we dug down and we found this arch from above and smashed through there. There was screams of joy. We’re in. We found it. We found it. It was the start of an amazing project.

 But the enthusiasts soon faced their biggest challenge. Williamson’s long-lost tunnels had been filled in with a mixture of rubble and junk dumped by the Victorians, which needed clearing out. glass bottles, jars, you name it, it’s down here. It had been used as a Victorian uh tip. The enthusiasts had no idea how deep the tunnels were, and with no surviving map, no idea of how far they ran.

 It took them 10 long years to win permission from local authorities to excavate them. But Chris’s colleague, Tom Stabledon, remembers clearly the day they started to dig. We had the first skip delivered on site on the 18th of November. I think it was 2012 and uh we spent four solid years emptying this place [Music] out until we finally reached the bottom and found ourselves 60 ft below ground.

 There was uh spoil piled right up to the roof virtually uh to the end of that chamber. Um, you couldn’t have climbed over the top of it and we had no idea what was beyond. In among the spoils, the volunteers were excited to discover treasures from the Victorian and Edwardian eras. One of the things I always pick on is that little mug there.

It’s definitely a coronation mug cuz it says 1902 on it that we know would be the coronation of Edward IIIth. And one day somebody chose to clean it out for some reason and found there was something in the bottom. didn’t know what it was. For no apparent reason, somebody thought to hold it up to the light and that’s what appeared.

 That is King Edward IIIth and he’s embossed into the China clay before the potter’s been fired. Since found out that it’s called lithophane work, it’s a rather nice little thing and it always terrifies me every time I pick it up in case I’m the one who drops it. Though the excavation is still underway, so far the team has discovered around 3 km of tunnels with three access points, but there is still so much to discover about why some of the brick work is even here. This is the arch to nowhere.

 Uh we call it the arch to nowhere because essentially as you come in, you think, “Oh, it’s a tunnel entrance going off to the left,” but in reality it’s not. I wonder whether it might have been a practice piece for apprentices or maybe there was going to be another tunnel going off into the quarry wall, but for whatever reason, it never happened.

Joseph Williamson was highly secretive about the tunnels. And why were these underground spaces created with so much meticulous brick [Music] work? There there is a a lot of mystery to Williamson because Williamson, as far as we know, never left any paperwork to say why exactly he did what he did. He was so secretive. Yeah.

 Didn’t boast about what he was doing. He didn’t talk to many people about what he was up to. It was just him and the men who worked for him borrowing away for 35 years. In the early 19th century, the UK was rife with unemployment. Some people in Liverpool believed that Williamson acted out of a spirit of philanthropy, offering work to men in need of a job.

Through Williamson and his tunnels, they were able to gain meaningful employment. They were able to take a wage home. They kept their pride. They were able to feed and clothe their families. As the excavation continued deeper, the brick vaulting gave way to the raw rock face of the original quarry, which showed the sandstone was laboriously cut by hand.

 You can see in certain places where the men have been hacking out with the hammer and chisel, and you can see it goes goes in different directions. That could be somebody who’s left or right-handed doing the the difference angle. It took the volunteers 4 years to clear out the northern section of the tunnels beneath an area known as Paddington.

 In 2016, they reached the bottom. I’m standing at about 60 ft below ground level here. By the time we finished, it felt like a brilliant achievement to have emptied this place out. At the same time for all of us there was a serious disappointment that we didn’t find any tunnels leading off this uh which we hoped would have connected up with the rest of the Williamson tunnel system.

But their disappointment was short-lived. Just around the corner stand the remains of the house where Williamson lived with his wife Elizabeth. The volunteers have made some tantalizing discoveries beneath it. This is Williamson’s house or what’s left of Williamson’s house. It’s the last remaining above ground structure that was built by Williamson that we know of.

We’ve been uncovering what’s below ground because there’s nothing left above ground to be seen. Well, I think the thing that that I found interesting was finding the four fireplaces with their complete with their ranges and of course finding stone carved skirting board as well, which shows that you are inside his house.

Beneath the cellar, they discovered a great void. No one knows what it was used for, but the volunteers have given it a nickname. This is the banqueting hole. Williamson’s had his men quarry out all the stone from this chamber. He’s used the stone for building and then he’s vaulted over it with this lovely brick archway and then he’s built his own three-story house right on top of it.

 Each week around 20 volunteers work here across the site. One of them is a recent arrival to Liverpool, an American, Caitlyn man. It started because I was like, “Oh, it’s just archaeology I can go do.” But now it’s just like this is my favorite place. These people are like family. So it’s just kind of like coming to hang out every Sunday and Wednesday with all my friends digging in tunnels.

The first time I went down there, it was just dirt. There was just dirt everywhere. That’s all it was. There was nothing else. And then each time I went down, it was just like what? Like there’s the walls are starting to show up. And then we were like, we found the floor. And then we were like, there isn’t a floor because now we have this new tunnel coming out.

 And so every time it’s just like this, like the span and the size is just crazy. The newest area of the tunnels to be discovered was found behind a brick wall. Volunteers on their hands and knees with buckets and shovels are here today to remove more soil. We’ve just come along to try and uncover the mystery of why he did why he did them, why he dug them, and where they go to.

 So, it keeps us digging. Every weekend or every week I’m here, it’s always something new to find cuz uh he’s left no plans. There’s no designs. This is the only part of his house that’s still standing. And um we’re here most weeks and keeps us going. It’s a good hobby. Mad, aren’t we? Well, I think we’re in the start of a much bigger tunnel, but at the moment, we’re right up in the roof of it.

 If this is the big tunnel, there may be another 35 ft to go down here. Without the hard work of the volunteers, these tunnels and their intricate arches may have never been discovered. It’s incredibly rewarding working on this. Um the the the tunnels are so unique. We just want to see as many of these tunnels as possible, made accessible for the people of Liverpool and everybody from all over the world to come and view them.

 It’s incredibly important to carry this on. Unearthing these phenomenal tunnels, the volunteers have opened the door to another world. Each week I come, we don’t know what we’re going to find. We could find a new tunnel. We could find a new like bricked in area of a room with tons of finds in it. You just never know. So that’s what’s so exciting.

There’s a lot going on, a lot more to find. We sometimes joke that uh somewhere buried there is a big trunk full of all of Williamson’s papers. But at the same time, it’s the fact that we have so little information that’s so intriguing. And I think for most of us, it’s what drives us on to keep looking for answers.

 The enthusiasts working hard underground to discover the truth. And the citizens of Liverpool above them may never really understand the enigma of Joseph Williamson and his [Music] tunnels. Slovenia, a central European country known for its soaring peaks descending into thousands of natural caves. Beneath its beautiful landscape on its northern border with Austria, a network of tunnels created by man unveils a rich history.

They uh fill it with explosive and they just blasted it. But out of the darkness, you know, when one thing died, the second thing was born comes a new world of exploration and adventure. We are really, really deep. 750 m down in the mine to navigate one of the largest man-made underground worlds by torch light.

 It’s the only underground kayaking route in the world. Why would mountain bikers want to venture into the mine? It’s not for beginners. Super technical, super steep. And how was their access made possible? The cave rescue team of Slovenia, they came and help us. So with some explosives, the tranquility of the Caravan mountain range is broken by the constant wor of spinning wheels.

 Mountain biking has become a high octane sport in Slovenia and it’s also gone underground. Inside the Petza Mountain lies a vast mining network once used to extract precious metals. Now it’s a breathtaking racetrack for adrenaline junkies like ANA. It’s not for beginners. This is for advanced and expert mountain bikers. This is not a place to try out your skills.

 Super technical, super steep sections going down with some small drops. Anna’s family has direct links to the history of the Mizicha mine. His grandparents toiled in these tunnels before it was [Applause] closed. Now ANA has created the only extreme subterranean bike route in the world, the Black Hole Trail. It was all handmade, no machinery.

 We asked for help the cave rescue team of Slovenia and they came and helped us also with some explosives on some areas to actually build the whole trail. Opened in 2017, the black hole trail was built in the abandoned tunnels of the Micha lead and zinc mine. The track runs for a staggering 10 km.

 When we started work, we actually knew nothing about working in the mine. So uh we always had the supervision and help of X miners because without them we couldn’t do anything like this. The 800 km mine network was abandoned in the 1990s but ANA dreamed of creating the most challenging bike trail ever to be made underground. Uh this is uh all limestone.

 If there would be an earthquake here, we would be safer here in the mine than outside in the buildings. The trail attracts thousands of intrepid riders from all corners of the globe. On the trail, you have different terrains like you have sandy terrain which is like quite slippery like surfing on a mountain bike.

 Then you have a pure rock which has a really a lot of grip all the time. Uh when you’re riding, you need to focus expect the unexpected uh at all times. These chambers lay untouched for 25 years. But there are still many tunnels that are too dangerous to ride. You have chamber by chamber, tunnel by tunnel. I think after that reach, you have quite a free fall and it keeps on falling.

 If you don’t know where to go, it’s pretty easy to get lost. The trail drops underground from one valley to another through five levels to a depth of 150 m. In Slovenia, this trail is a only trail. Going from one point to another point without going on it twice. More times I do it, more fun I have.

 All the other caves in the world were made by nature, but this cave was made by the humans. When the bikers leave and the sounds of racing tires subside, the last vestigages of the mining equipment sit in silence and darkness. The mine dates back to the 1600s and was a rich source of lead and zinc. More than 800 km of tunnels were excavated for its precious metals.

For decades, geologists like Yuros Hellet have continued to explore the mine’s network to better understand the makeup of the bedrock. They’ve discovered just how stable the tunnels and shafts are. The first time I visited this place was 35 years ago and actually it was a field trip.

 The area is built out of uh very stable rocks and that means that you can visit sometimes uh tunnels which were built 300 350 years ago. In over three centuries of mining, 19 million tons of lead and zinc ore were excavated from the tunnels. Digging was discontinued at the end of the 19th century. The uh most important mineral in this area is this dark uh mineral with metallic luster which is lead sulfite.

Um this is zinc sulfite. It’s a little bit brownish sometimes greenish yellowish. Lead and zinc and their derivatives appear in many everyday objects. Lead can be found in roofing materials, car batteries, and ammunition. While zinc is often used in alloys, paints, cosmetics, and plastics.

 Massive effort was required to extract these minerals from the rock walls. They had to drill uh a hole uh for let’s say uh to half a meter, 1 m deep. They uh fill it with explosive and they just blast it. By the early 1960s, the Mazicha mine employed more than 2,000 people. Whilst he was still at school, Mame Proch worked here 2 days a week.

 So see here here you see the uh different roads, the different tunnels in the different ways and this is because we builded this tunnels for uh researching. So we search for lead with the tunnels. It was very interesting job and each time when I came to the work it was not the same place, not the similar place. Each time was something different.

 You know a temporary drop in world prices for metal and rising mining costs brought production to an end in 1994. On the last day uh when we work in the mine uh the worker were sad because we shut the pumps down the mine is start getting flooded with water and that kind of stuff and uh of course it was sad but uh you know when one thing is go died the second thing is born.

 Miran is proud of the mine and its achievements. He takes great pleasure in showing visitors around the extensive tunnel system. It’s interesting for the people who visit us because they don’t expect to see something like like the mine. In Egypt, they build the pyramid, but we build the mine over here. [Music] Following the closure of the mine, the local community began developing another industry, geourism.

At the 1997, we opened the first part of the touristic mine and museum. Now around 40 km of the mine can be visited by tourists. Exploring by miners train or on foot, they can see the great caverns excavated by hand. This mine is all time alive. Relics from the mine’s industrial past evoke memories of a bygone era.

After the Mazicha mine was closed, water was no longer pumped out, allowing the lower chambers to flood, creating magical underground lakes. This is the only underground kayaking route in the world. For those who want to delve deeper, Lara Pico takes people on unique kayaking routes. It’s pretty special to come here every day and just to see it because it’s something really unique and not a lot of people get to experience that.

 Every time it’s different, every day it’s different. The water level is not the same every day. So, you never know what to expect. In these caverns where men once toiled, now a network of rivers flow, carrying 6 million cubic meters of fresh mountain water. The water is really clean, and I’m going to prove it to you by drinking it.

[Music] but cold. We are around 750 m below the surface. So, we are really, really deep down in the mine. You can see the railroads because there used to be a train going through the tunnels. There are 3 and 1/2 kilometers of underground waterways for visitors to navigate. You can go explore it on your own.

 You can go through the low tunnels, through the big empty spaces, the galleries, and we also have a small rapid in here. And for the braver ones, you can also jump in. I never fall off. The water is too cold. So, I am always very careful. Since 2013, over a quarter of a million tourists have visited the mine and more than 20,000 have descended on their bicycles.

So, it’s really interesting because along the way you can find a lot of different stuff. You can find minerals. You can find, of course, the ore. You can also find old signatures of the miners that used to work here. So, the whole history of the mine, you can actually see it in here. Conditions that are in here are extreme.

 The cold, the water. I really admire the miners what they did. Rich in history and precious metals, the Msicha mine has now become a paradise for extreme sports fans. This really is an exhilarating underground world. [Music] [Music] under Scotland’s capital. How did these tunnels become a den of criminal activity? The illegal trades decided, why are we not using this

space? It’s vast. It’s dark. It’s perfect. Underneath Italy’s busiest port. Why did these ancient aqueducts and royal escape routes become a haven during World War II? This was about something good. They survived. And what Cold War relics are hiding in plain sight beneath Finland’s first city.

 This swimming pool works as a shelter for 3,800 people. Beneath our feet lie extraordinary spaces, caves, and tunnels. The span and the size is just crazy. They’ve been designed and built by us. This is the only one with a castle as well as formed by nature. But how were they created and adapted? By who and why? You’ve got to face your fears.

 Throughout history, subterranean life has captured our imagination. Feel so privileged. We’re going further and deeper to unearth the mysteries, the stories, and the secrets of underground worlds. Scotland’s capital, Edinburgh, a city rich in heritage. But few visitors appreciate that many of its fine Georgian streets stand on top of a network of hidden bridges.

 The bridge was built to span the cowgate valley under the bridge as it were. A lot of people get confused really how there’s so much space down here. Within the bridge, a labyrinth of cellers and passageways that lay forgotten for over a century until an amazing discovery. So I thought, “Oh, more vaults.

” It was quite exciting finding all these pieces. But those vaults now reveal a sinister side to the city’s past. They needed a constant supply of human corpses to keep dissecting them for their students. She had the rise of body snatchers, grave robbers. Scotland is the UK’s northernmost country and its capital Edinburgh is home to nearly 500,000 citizens.

 The city stands among hills and valleys. When it was growing in the 18th century, the citizens refused to be limited by the geography. They built bridges to join the hills together and the first one was across the cowgate valley. Southbridge took 3 years to construct. It’s 300 m long. When it opened in 1788, a whole shopping street was built along the top.

 Simon Bendle has been guiding visitors around the arches and subterranean spaces within the bridge for 3 and 1/2 years. The bridge was built to span the Cowgate Valley to link the old town in that direction to the to the new suburb supported by 19 large arches. Originally those arches were open so people could pass freely back and forth underneath the bridge.

 But quite quickly buildings, tenement buildings were built either side of the bridge, sandwiching the bridge and then and then closing the arches. These were homes for the growing population and some were built to nearly the same height as the shops on the bridge. So on the South Bridge shopping street, you would have no idea that you were actually standing on a bridge.

 In the 18th century, the now hidden arches were subdivided into vaults of various sizes. The plan was to rent out the vaults and use the money to pay for the upkeep of the bridge. [Music] These chambers were originally used as storage areas for businesses on the bridge above, shops on the bridge above and also as workshops.

 So people like jewelers and bookbinders and leather workers. There are at least 120 of these vaults. Jamie Costine, a Southbridge expert, has spent 9 years learning about their history. They would make them workshops. They’d floor the big arches up to three times and then split them in half again. They ran corridors to connect them all.

It would keep a source of income and they would keep that money to help for repairs and maintain the bridge. It was a great idea. It really was. For 30 years, the vaults worked well. But South Bridge was built on the cheap. What was once a bustling network of studios and warehouses quickly descended into a living hell.

 The bridge leaks from day one. The water sits on the top and it sits on the surface and it seeps through. As you can see up here, we have the stallites. This is caused by the water dripping through from above. And this wouldn’t have been fresh rain water we’re talking about. This would have been water infused with horse droppings from the bridge above and the contents of those uh nasty buckets as they called them, the slops buckets that the people used to throw out the window at night into the street.

 This raw sewage gradually seeped and dripped through the bridge, creating stallctides of effluent. The allervading damp turned the vault into a ratinfested disease-ridden slum. Because of this damp, disgusting conditions really, businesses quickly abandoned these vaults. 160 years later, the former Scotland rugby star Nory Rowan accidentally became an expert on this underground world when he bought one of the old tenement buildings that backs onto the south bridge.

 I mean, Edinburgh is known as the city of seven hills because it’s a city of seven hills. It’s a a city with lots of bridges connecting all these hills. In the 1980s, while Nori was converting his tenement into luxury flats, he discovered a blocked doorway which led into the vaults. Started renovating the subb started digging everything out.

 So then I found a tunnel which led into the first of the vaults which made me realize there was more and more vaults. And here you see some stallag tites. When I first broke into some of the chambers, the stall tights were that big. They reach from the the ceiling to the floor. Nori didn’t stop with this first discovery.

 He continued to delve deeper into the system where he found rooms mostly filled with rubble. He dug them out one bucket at a time, not knowing why the rubble was there or what rooms lay beyond. I found all this area in here. These all these little bays here were full of straw. Yeah. And this this was virtually untouched from I don’t know the 18th century, early 19th century.

 I used to come every morning and just dig for two or three hours. It was quite therapeutic. Shavers Weekly, which was a students magazine at the time, they they nicknamed me Mole Man. Nori now runs an entertainment venue and bar in the spaces he’s cleaned out. But why were the vaults filled in with rubble in the first place? The answer lies in the early 19th century.

 When the shopkeepers and artisans moved out of the damp vaults, Scotland’s criminal underbelly saw an opportunity that was too good to miss. To ply their illegal trade away from the prying eyes of the law. The illegal trades decided, you know, why are we not using this space? It’s vast. It’s dark. It’s perfect. And when the good guys moved out, that’s when the bad guys moved in.

 And then the vaults were used by gangs of thieves, people running illegal whiskey steals, um, women pursuing the oldest profession. Anything that was illegal above ground was perfectly legal within the confines of the structure of the bridge. There was no police here. Because of their cheapness, the vaults also became a living space for citizens who had fallen on hard times.

Desperately poor people would have taken shelter down here. This was a time when people were pouring into Edinburgh from the highlands uh and from across the water in Ireland. So whole families, the poorest of the poor would have lived down here in appalling conditions. And that continued right onto the 1860s.

In amongst the poor families and petty thieves, there operated the most feared criminals of them all. And that’s when these vaults were handed over to the criminal types, the bootleggers and the body snatchers. In the 1830s, Edinburgh was renowned for its medical schools, and a grizzly new trade sprang up. Enterprising criminals would dig up newly buried corpses from their graves and sell them to student doctors so they could study human anatomy.

If you could deliver a body to a doctor in a good condition, you could get perhaps £10, which are years wages for a for a labor at the time. They needed a constant supply of human corpses to keep dissecting them for their students. Um and and the law at the time said only the bodies of executed criminals could be legally dissected.

 So you had the rise of body snatchers, grave robbers, or as they prefer to call themselves, the resurrectionists to avoid the risk of arrest by the police. Body snatchers worked in the dead of night. That what grave used to do is strip the bodies of any shrouds or clothes or any wedding rings because they feared they could be charged with theft.

 Uh that was a capital crime. you’d hang for that. But if you were caught carrying a naked body around, it was more of a kind of weird new gray area. You’re more likely to be just charged with a lesser crime of desecrating a grave. In the early 19th century, the Southbridge vaults were apparently a hunting ground for the infamous criminals, William Burke and William Hair.

 They embraced the crime of selling dead bodies for cash and took it one step further. Uh the curious thing about Burke and Hair is they were neither Scottish nor were they body snatchers. They came from the north of Ireland in search of work. I suppose you could say they were innovators. They cut out the hard work of digging up their bodies and they just killed people instead. So they were serial killers.

The killing of 16 people in 10 months and Burke’s execution and public dissection as punishment shocked respectable society. The notoriety of the Southbridge vaults in this case meant that respectable Edinburgh would rather see them forgotten. By the 1870s, they’d been filled in with builder’s rubble and consigned to history.

 Despite Nor’s extensive clearing of the vaults, there is still much to discover. Kept finding more chambers and I just kept it was quite exciting finding all these pieces. I kept thinking I was going to find the crown jewels or something really valuable. This is a tiny tiny section of this entire structure. When it first opened, it really was an underground labyrinth of tunnels and chambers.

Norris taking Jamie into one of the deepest areas of the Southbridge vaults. So, is this the original point of entry? It was through here. Yeah. It’s an area which Nory has dubbed the well. When I dropped the floor level here, lifted a slab here. As you can see, I found this big hole and there’s a couple of tunnels going off to the left there and the right.

 This shaft drops for at least 10 m and suggests a lower level that yearns to be explored. I’ve never been along there, so I just uncovered this big hole, but I had enough work on my plate at the time and never did anything about it. So, I’ve just covered up and built this we wall around it, stopping anybody going down it.

 So, I suppose the next thing is to start having a we look and see what’s actually down there cuz these ledges. Have you got any idea what these could have been from? Cuz they’re different. No, I think that’s just lintils spanning over the tunnel. As you see, there’s a tunnel goes off to the left there. Yep. Now, it doesn’t seem to be a well with all these tunnels. Well, exactly.

 Cuz wells don’t have archways in the walls and wells are not paved at the bottom, which is that is clearly paved. While there are still areas of the labyrinth waiting to be explored, most of the vaults have been given exciting new uses since Nori rediscovered them 30 years ago. Music venues, comedy clubs, and even wine bars paying homage to one of the original uses of the vaults.

 It got a nickname of Whiskey Row. Um, in 1815, one of the illegal illicit distilleries was busted in here, this basement here. Despite the troubles along the way, Edinburgh’s South Bridge is one of many great structures that have transformed the city. It is a brilliant piece of engineering and it’s full extent.

 I don’t think we’ll ever ever find out or comprehend just how massive this place really was. The underground spaces here gave the city’s inhabitants both shelter and suffering. I think the reason why I find them so interesting, these vaults, is simple as they look, they’re kind of like a window into the social history of Edinburgh.

 The stories of these spaces still capture people’s imaginations to this day. Well, I think it’s great that they’re still being used here. They are 200 years after they were built and they’re still in use for something and they’ll probably keep reinventing themselves as time goes [Music] on. Italy in southern Europe, a land of illustrious history reaching back to the Roman era.

The south of the country was also inhabited by ancient Greeks whose influence can still be seen to this day. One of the country’s largest ports is Naples, 230 km south of the capital Rome. And close by the ancient but still active volcano Mount Vuvius. The old city was built from volcanic sandstone named tough or tufo as the locals call it.

 They mined from the geothermal bedrock, leaving behind these incredible underground spaces. The city was built thanks to this rock. But how did the ancient engineers achieve these feats of excavation? It’s like a uh a parallel town in the underground. What uses are being found for tunnels which date back more than two millennia? This was a tunnel underneath the city for royal families.

 And how many more underground worlds are to be discovered? Wow. [Music] Naples is a thriving Italian metropolis, but it was founded by Greeks in around the 8th century BC. They colonized the southern coastal areas and their lands were known as Magnagracia or Great Greece. Naples was one of their foremost cities. Having dug out the volcanic tough to build above ground, the Greeks were left with a 450 km network of tunnels running 40 m beneath the city.

 The Greek labyrinth is known as the Napoli Sautterania. Enza Tini, who has a PhD in architecture, has been studying this underground world. The first time I came here, it was really extraordinary because here you can see exactly what the Greeks were doing when they were excavating. The Greek were finding some cracks into the tofu stone and putting piece of wood in the middle and uh creating the void around it and and taking the material for the building upstairs.

The Greek excavations in the 8th century BC were the beginning of subterranean life beneath Naples. It may seem risky to build an underground system so close to a volcano. But the Greeks did all this around 800 years before the famous eruption of Mount Vuvius in 79 AD and nearly two millennia before the geological risks were understood.

 The Greek underground system was developed further by their successors the Romans. [Music] you find traces of Romans that were with the engineering system were creating a city. So I really think it’s a magic place. After a series of battles, the local Roman tribes reclaimed the territory from the Greek colonists in around the 4th century BC.

 Among their many engineering achievements, they converted the Greek tough mine into a bespoke underground freshwater system of channels and wells, some of which are now mapped and accessible to visitors. This water system was supplied by an even bigger one, a massive underground aqueduct, which brought in water from a mountain spring 70 km away.

 It was known as the Aqua Augusta. The Romans used it to supply nine major towns, including Pompei. The aqueduct was named in honor of their emperor, Augustus, and the entire system was nearly 150 km long. Local journalist Marco Poro is meeting Enza to learn some more about the historic water system. The activate we are in is very very ancient.

 I think it’s about uh fifth century uh before Christ and all the ancient aqueducts were uh connected uh together. You don’t imagine that you can go down and find this quiet mysterious place. They start excavating and found this great, you know, stone that could be excavated easily and in a safe way, creating this void. The Roman engineers lined the aqueduct with plaster to seal it to stop water leaking through the porous tough.

or Romans discovered that, you know, just putting some plaster on the tufa stone could be a a collection tank for the rainwater collection and they start to connect everything, all those boys to the ancient abolute. This great underground project beneath Naples holds a unique place in the hearts of its citizens.

The underground for Neapolitans. Uh we love this place because here we can find the full history of the town from Greeks, the Romans, by the aqueducts. I think here the time is frozen. It’s like a a parallel town in the underground. The vast infrastructure is still intact. This network continued to supply Naples with water until the 17th century when it was eventually replaced and slowly became disused.

 But not all the subterranean spaces are unused. Marco has come to see an experimental project researching spaces formerly used for water supply to farm without any water. Wow, that’s wonderful place. That’s incredible. This is a place full of humidity. That’s why the life is possible underground. We have an experiment made by the local university uh in which uh we can see plants growing up without water only by humidity and electric light.

 We don’t need sun to have life underground. We don’t need water. You can see these plants are growing beautifully. Thanks to projects like this, Italy is now among the group of countries like the UK, the USA and China using subterranean spaces to feed their ever growing populations. Basil is very very important for Neapolitans.

 We use basil in the pizza for example or in the pasta that’s very very particular and it has a a good taste. We can try it. H pretty good. The volcanic tough beneath Naples continued to be relevant to the city’s fortunes long after the Acroagusta was abandoned. In the mid 19th century, another underground project was begun.

This was a tunnel underneath the city for a royal family. So everything must be beautiful. This tunnel was originally intended for the exclusive use of Naples ruling family, the Borbin dynasty. Taking their name, it’s now called the Galleria Bonica. But it was to play a far greater role in the lives of everyday Neapolitans almost 100 years later during the Second World War.

 Elellanena Corino is one of the expert guides. The Bourban tunnel is 430 m long, 5 years to build this place which was completely handmade. But why was the tunnel created? Italy in the 19th century was a mass of rival states, but there was a popular movement towards a unified country. Riots in Naples caused the emperor Ferdinand II to fear for his survival.

 So in 1853, he ordered a tunnel to be dug from the royal palace to the patza Victoria near the sea as an escape route and to deliver a defense force if necessary. Like the ancient water system, the Galleria is 40 meters beneath the surface of the city. But most Neapolitans have little idea of what lies beneath their feet.

 I discovered that this place is amazing. It’s full of history. As you can tell, it’s so quiet. This is thanks to the rock. Uh the tufo, it’s a soundproof rock. So basically, you can hear the city, all the noises. From 1853, workmen digging by candlelight spent around 3 years on the tunnel, but it was abandoned once Ferdinand’s dynasty was overthrown.

 His territories were eventually unified into the new kingdom of Italy in the 1860s. The tunnel lay disused for 80 years but found a new use during World War II as a bomb shelter and the Italian army improved the access for civilians. As part of World War II bomb shelter plan, the soldiers uh provided different entrances and this was one of the five with 100 steps.

 It was really narrow, but Italian soldiers enlarged all the tunnels so people could just run and reach a safe place. Naples was the most bombed Italian city during World War II, suffering 200 air raids in 4 years. Allied forces were desperate to disrupt German supply lines to North Africa, and whole areas of the city were reduced to rubble.

With almost 10,000 homes destroyed in the city above, the Galleria now provided life-saving shelter to stricken civilians. We know that there used to be a medical aid zone as people could get injured. Uh there were areas for pregnant women because we know that some children have born in here.

 There were areas with beds because people could sleep in here during the night. Historians have made fascinating discoveries about the living arrangements in the Galleria during World War II. When we found this place, we just thought how people could be in here. I mean during the bombings, but also living in here if the house was destroyed.

 We found um many double beds, but each one was not used by a couple like nowadays. Four, five people, maybe six. One family with one beds. Historians were surprised to find graffiti probably left behind by a civilian who sheltered here during the Allied bombing campaign. And we have a special writing in here. Noivi. Noivi means we are alive.

 And this is the most beautiful writing that we found. This was about something good. They survived. People during the war have been saved thanks to these places. For more than 2,000 years, Vuvius’s volcanic rock and Naples has been excavated to create ambitious subterranean spaces. In 1986, work started on a new underground railway.

 Naples now has more than 20 km of metro line serving 22 stations. Art historian Maria Corby has an indepth knowledge of the newly built art stations as they’re known. An exciting architectural project. The underground in Naples was uh very important to transport people from one place to another and today it’s still important because underground uh we have the metro trains.

[Music] International architects were entrusted with designing new stations to make these public spaces aesthetically pleasing. It’s another instance of the ingenuity shown by Neapolitans over thousands of years in their attitude towards underground worlds. Gary Baldi is Naples central station.

 The work of the prestigious French architect Dominic Perau station is 15 m deep and the light comes down till the platform layers. Uh this is maybe uh the more um contemporary station. A lot of mirrors and [Music] steel. Two stops away is University Station created by Anglo Egyptian architect Karim Rashid in 2011.

 The vibrant artworks are based on continuous profiles of human faces. But they also conceal structural [Music] pillars. Toed station is the deepest on the line at 55 m. The work of Catalan architect Oscar Tuske Blanco. Inspired by the shape of a volcano’s crater. The design incorporates conicle shafts that bring light deep underground from wells at street level.

It was inspired by Bizuvius, the symbol of Naples. And this is the crater of light. Color is used throughout the station to suggest the layers of the city. As the passenger travels down, it changes to signify descent from ground level down to below sea level. So, uh, from the warm colors of the tough, we are going in the blue in blue mosaics of the sea.

The confidence expressed through these innovations is only the latest chapter in the city’s story of underground worlds. Around 1 million square me of space beneath the city has been mapped, but it is thought that there may be a further 2 million yet to be discovered. Surely the city should think about how to map all these voids.

 I I think this place is very special for us, you know, a heritage that we have to keep safe for the [Music] future. Finland, a Scandinavian country in the north of Europe between Sweden, Norway, and Russia. The capital Helsinki is now using relics of the past to create incredible underground worlds for all its citizens.

 How have the ingenious Finnish people transformed their Cold War shelters to give them a new role in the 21st century? We have over 900,000 shelter places, more than we have citizens. What exciting uses have been found for the disused emergency bunkers? This is our track. It’s about 300 m long. And here is our long straight where you can get up to the top speed of 60 kph.

Finland’s capital, Helsinki, has been developing an underground master plan for the entire city, reusing older subterranean spaces and creating new facilities for the population. Here in Helsinki, we have actually created 12 million cubic meters of extra space underground. Geologist Eron Udman has been working since 2012 on some of Helsinki’s most important underground structures.

The dark yellow, these are locations where we can build other caverns. The city is built on granite, which makes excavating large spaces underground relatively safe, inexpensive, and sustainable. Granite is really strong, and that means that we can blast holes and make excavations in it. Finland started to build its underground worlds in the 1960s.

 The country was neutral during the Cold War, but they excavated hundreds of underground bunkers to shelter their citizens in the event of an attack. It’s a unique feature that we have here in Helsinki that we have so many tunnels. Uh and and if you compare that to other countries, this is really in Helsinki is very special.

 In the city area, we have over 900,000 shelter places, more than we have citizens. Andrea Schneider is the head of emergency preparedness at Helsinki’s city rescue department. He’s responsible for hundreds of cold war emergency bunkers hiding in plain sight all over the city. It is my responsibility to put the plans for the civil defense and the protection plan of the city of Helsinki.

 These rock shelters that are inside the rock, we have over 50 in Helsinki area. The biggest ones are for 10,000 persons. As cold war tension subsided in the 1990s, Helsinki found imaginative ways to use the underground bunkers whilst keeping them operational, combining defense capabilities and community facilities would later become the underground master plan.

 The shelter system consists of independent shelter places all around the city and they are in a double use. So we are trying and and planning to use them in normal situations as widely as we can. So they are not just laying empty or as as a storage rooms. So they are actively in the use of the society and the and the citizens.

 There are over 400 facilities around the city some of which can be transformed into shelters in 72 hours. This one has been incorporated into the largest leisure center in Helsinki, the Itakasca swimming pool. This is the absolutely the biggest one in Finland. About 1,000 people visit this place every day. Oleg Yaon began working here when he was a lifeguard and is now in charge of daily operations.

 A swimming pool underground has obvious benefits in a country with cold winters. Now we are inside the rocks. So the heat are staying inside. So we don’t need to heat this place much. So we are saving energy because we are underground. This swimming pool works also as a civil shelter for 3,800 people. This rock is really solid here. So in in in Helsinki, this was one of the best places to build the shelter and the pool.

 Divided into two levels, it can provide safety, food, and water for up to 14 days. The shelter’s capabilities have been incorporated into the design, including the maintenance floor below pool level. Constructed so that we have at least 50 m uh of the bedrock on top of us. So then the doors will take all the pressure. In the event of a nuclear attack, there are two doors operated by hand.

 The first takes the initial impact from the air pressure protecting the people inside the shelter. The second is a 5 cm thick steel door to prevent any radiation and contaminated air from entering. We have the doctrine of of defending the whole country and defending all the civilians and not to evacuate them.

 And in Helsinki when we are not planning to evacuate the people, we have to protect them where they are. So, we’re trying to keep the city functioning at all situations. Reusing existing Cold War shelters throughout Helsinki is a major part of the underground master plan. Around 3 km away from the swimming pool is the formula center.

 It’s another bunker that’s been given a new purpose, this time as a go-karting rink. And Yuka Manpa is the manager. We are actually at the bomb shelter but we are renting this place from the city of Helsinki. We are about 20 mters underground and we run a co- cutting place here since 1995. We get about 10 to 20,000 customers per year.

 You don’t have to worry about the weather here and well since we are using wasted space. Our rent is pretty cheap. This is our track. It’s about 300 m long. And here is our long straight where you can get up to the top speed of 60 kph. Our cars run on petrol. We have pretty uh strong air conditioning here. You can imagine the fumes.

 We get rid of them pretty quickly. Like all Helsinki’s dualpurpose facilities, the formula center can be rapidly switched into shelter mode in the event of an emergency. If we received an emergency call, we would have 24 hours to get rid of all of our stuff. All the locals, about 5,000 people, would come here and be safe from whatever emergency up there is.

Each of these spaces was built to meet civil defense needs. Yet, they have fantastic dual uses. Even so, they are not the only cold war surprises to be found in Helsinki. One of the city’s architectural masterpieces is the Templio Church, also known as the Rock Church. It was designed in the 1960s by architects who were infused with civil defense concerns of the cold war era and the need to shelter Helsinki citizens.

This is a church that dates back to 1969 when it was built. Heli Suhala is an expert on the design of this Lutheran church which burrows 13 m down into the granite bedrock. the architects these two brothers Timo and Tuo Swamalinan have been working for the army earlier so probably that’s where they got the idea from to build into solid rock they blasted really their way into the bedrock which is millions of years old and you can still see the marks on the walls where they have been drilling and blasting and they wanted to keep it that

way because they wanted people to see that this is part of nature Beneath the copper dome, the rough huneed rock walls were left bare on the advice of a leading acoustic engineer to enhance the space for musical performance as well as worship. Apart from being church, uh we also have around 400 concerts per year.

 The acoustics here are fantastic because the surfaces aren’t smooth at all. Given that this beautiful church was designed at the height of the Cold War era, it should come as no surprise that beneath it lies yet another nuclear blast shelter. It was of course highly important in those days and even today actually in a time of war the bomb shelter downstairs would be used.

 So in a time of crisis, people could live down here at least for a period of time. Nowadays, we use this as a backstage area for our orchestras and choirs performing here. The Helsinki master plan encompasses underground worlds from more recent decades. The weekend Macki waste water plant operates 20 m beneath the bustling city streets and opened its slle gates in 1994.

 Paula Lindell is one of the managers. This is the longest carrier in the treatment plant. We use bicycles and electric cars uh to go around as it’s it’s quite a long long distances for the workers. It’s the largest sewage facility in Northern Europe and processes around 270,000 cubic meters of waste water each day. It’s 14 hectares underground space.

 It’s around 20 times of the football field. So, it’s quite a good use of space. Also, the design allows for more than double the regular capacity, up to 700,000 cubic m of waste water. The need for this could occur during severe winters with sudden thors of ice and snow. So, here you can see the layout, the water process part.

 The blue part is underground. It’s underground because in Finland we have quite a cold winter and also we have really hard rock. So it’s uh um easier in Finland to go underground than in some other places. And the temperature is constant when you have underground plant. So it’s easier to maintain all of the equipment. The plant is designed to retrieve and recycle energy and organic matter for conversion into power and fertilizer.

The first stage involves screening out large non-organic objects which cannot be processed. All of the stuff have have come with the waste water and in the screening part we have taken it out of the wastewater. So a lot of stuff that should not ever enter the sewage network. dentures, spectacles, cell phones.

 How they entered the sewers is open to speculation. Of course, toys. So perhaps children may have put it into the toilet. I can only guess where it came from. Once these large non-biodegradable items are filtered out, the water flows through two stages of treatment. The first involves separating materials that can be turned into compost and bio gas.

So we are more or less 25 m underground and for this process there is seven similar size lines. So it’s 17th of the waste water going through this process tank. The waste water left over is sent for final chemical treatment before it is fit to be released back to mother nature.

 This is where the magic more or less happens. Here we have the biological process where we have bacteria working and cleaning the water. They get nitrogen out of the waste water and also organic materials and uh we have nine different lines for this part of the process. The Vikramagi plant handles vast quantities of sewage. It replaced 12 other treatment works around Helsinki.

When the waste water is cleaned here at the treatment plant, after that it goes to one big tunnel and it goes 8 kilometers uh below the Helsinki city center and after that also 8 kilometers to the open sea and it’s mixed to lot of volume of the sea water. The treatment results are world class here and uh when the uh treated waste water goes to the sea, it does not have an effect on the environment.

The architects and planners of Helsinki have designed their subterranean spaces initially to protect the population and in more recent years to wow them with high concept experiences. It is almost an underground city if you think about that. We are looking where are possible locations where we can build more underground space for all kind of of activity.

 And with many shelters now fulfilling dual roles, the Fins have the best of both worlds. We are looking into the future. [Music] [Music] on England’s coast. How did a secret wartime bunker save thousands of soldiers? And it was only in the 1980s that we realized what an extraordinary history they have.

 In these Dutch tunnels, what spine- chilling discovery shocked the miners. All of a sudden, you find this terrifying skull and they were frightened these people. And in Eastern Europe, the world’s biggest labyrinth, what secrets are now being revealed? They didn’t want anybody knew about the truth. What happened to Euro Civil War? Beneath our feet lie extraordinary spaces, caves, and tunnels.

 The span and the size is just crazy. They’ve been designed and built by us. This is the only one with a castle as well as formed by nature. But how were they created and adapted? By who and why? You’ve got to face your fears. Throughout history, subterranean life has captured our imagination. Feel so privileged. We’re going further and deeper to unearth the mysteries, the stories, and the secrets of underground worlds.

 The United Kingdom, an island nation that’s always kept a tight grip on its borders. During World War II, the White Cliffs of Dover on England’s south coast were on the front line in the face of a likely Nazi invasion. Underneath Dova Castle, And behind the cliffs, an extraordinary underground world was created to protect the UK from the Nazis.

 With this bunker becoming the control center for the Allied evacuation from Dunkirk, this is as good as it got for a bomb-proof headquarters. What made the Prime Minister Winston Churchill order the construction of these secret underground coastal defenses? And why did Adolf Hitler order his bombers to spare Dova Castle even as they reign destruction on the town.

 These chalk cliffs 106 m high overlook the Straight of Dover in southeast England. They form arguably the most famous stretch of British coastline facing out towards northern France which is just 35 km across the English Channel. There’s a long history of tunnels at the White Cliffs mostly under the famous Dover Castle a significant defensive landmark since the 11th century.

Historian and author Jonathan Code has spent more than 40 years researching the castle and its underground worlds. Do Castle has a very long history as a garrison fortress since 1066 right through to 1958. The castle has a three-story complex of tunnels beneath it. Two of those date from World War II, but the earliest level to be built was Casemate 15 m below the surface.

 It dates from the late 18th century when the UK feared an invasion by the emperor Napoleon’s French army. Do’s garrison of soldiers was increased and because the castle ran out of space, the extra barracks were built underneath it. These original tunnels were begun in 1797 at the peak of the Napoleonic Wars when there were about 2,000 troops.

 Men could have lived down here for months. Once those British troops defeated Napoleon in 1815, the Casemate Barrack tunnel was no longer needed. It lay abandoned for more than a century. In 1938, with World War II inevitable, the Royal Navy brought it back into use as a command center.

 15 m below ground, these tunnels would be able to withstand any German aerial bombing raids. it would be a very very difficult target. So this is as good as it got for a bomb-proof headquarters. [Music] From 1940 to 1942 and with Britain in the grip of war, the Navy wanted to expand the base under Dover Castle to bolster the country’s defenses along the south coast.

 They added two additional stories. First annex and then 50 m deep dumpy. The prime minister Winston Churchill wanted the best man possible to run the whole operation. He persuaded a respected first world war veteran out of retirement, Vice Admiral Bertram Ramsay. Admiral Ramsey, he was extremely good organizer and the ideal person for getting this naval headquarters running again.

 During May and June 1940, Ramsay’s staff often found themselves working 36-hour shifts as they tried to organize and direct one of the most daring rescues in history, Operation Dynamo, better known as the Allied retreat from Dunkirk. This is a very important part of Britain’s 20th century history. The evacuation from Dunkirk was planned and operated from here.

 The work here was absolutely vital. Operation Dynamo witnessed the emergency evacuation of 338,000 British and Allied troops from the French coastal town of Dunkirk after they were surrounded by the advancing German army. 933 ships were involved. The Dova control room was vital with hundreds of staff involved in directing the evacuation fleet.

 In adjoining rooms, extra staff were tasked with teleoning defense instructions to soldiers along an 80 km stretch of British coastline with their orders for our artillery defenses. We’re now standing in the coastal artillery operations room here, uh, which controlled gun batteries from as far west as Hastings right round, uh, to the South Fland.

 But with the advent of radar, with good radio and telephone communications, you get for the first time, you can get a whole sequence of coastal defenses taking the orders from one central point. Early in the war, Adolf Hitler let it be known that he wanted Dova Castle to be his headquarters when he conquered Britain.

 He instructed his army to avoid bombing the castle. But the town below was not so lucky. But following Dunkirk, from July 1940 onwards, German gun batteries in France started shelling the English coast. At the same time, the German air force, the Luftvafa, began its fight with the RAF for control of Britain’s skies.

 This fivemonth bombing campaign became known as the Battle of Britain. Dozens of British towns and cities were bombed during these German raids. More than 200 civilians were killed in Dova and 10,000 buildings damaged. The additional annex and dumpy levels at do were built because of these new hostilities. The annex was meant to provide emergency services to the towns folk.

 This annex was constructed u in 1941 to provide an underground hospital and there’s a very real fear that saturation bombing and possibly cross channel shelling would have led to very high casualties in the area. The annex is just 10 m below the surface. The hospital contained an operating theater and there were also mess facilities and a kitchen.

 We are actually standing in the reception area here where you would have had ambulances able to bring uh the injured people. Here you can see it’s very much more utilitarian than the casemates level below, but they were built for reasons of economy and speed. The lowest level of the do complex is called dumpy, supposedly an acronym for deep underground military position yellow, and any significance in the color yellow is now unknown.

Jonathan has been granted privileged access to Dumpy, which is currently off limits to the public. We’re now down in the deepest of the three sets of tunnels here. This is Dumpy level and approximately halfway down the cliffs in terms of uh depth. The intention of these combined operations headquarters was to have a forward command post from where the invasion of Europe would be controlled.

In the event, Dumpy was hardly used during World War II because the D-Day Allied invasion of Europe in 1944 was directed from Portsmouth instead. I don’t think these steps have meet any safety levels now in terms of landing stages to stop you tumbling too far. With the exception of the dumpy level, the Dova Castle control center has been restored to look just as it did in the 1940s.

 There’s another wartime shelter nearby that has undergone restoration without period fixtures and fittings. 8 km east of Dova Castle at Fan Bay stands the remains of a gun battery. In the 1940s, it would have looked something like this Germanbuilt gun battery on the Channel Islands. There was a group of concrete platforms on the clifftop, each with a 6-in anti-aircraft gun at its center.

 But the fan base site was demolished and buried nearly 50 years ago. The underground shelter beneath it was dug out by enthusiasts in 2014. Gordon Wise is a volunteer guide here. He knows the story of why it was built. Churchill on a visit here in July 1940 sees a small convoy of ships going through the Do Strait and he asked Admiral Ramsey if it was a British convoy and the embarrassed answer is no, it’s a German one.

 Churchill got very very angry with that response. Up until 1940, German boats could sail close to the British coast, knowing that there were insufficient defenses to attack them. Faced with such a major hole in the country’s coastline defenses, Churchill ordered the immediate construction of garrisons along the length of the south coast.

 The Fan Bay gun battery and shelter were built in just 100 days. Construction started in the November of 1940. Fan bay went operational St. Valentine’s Day 1941. The clifftop battery would have been manned by a garrison of 185 men and four officers. They were billeted in the deep shelter 23 m below.

 From gun batteries and pill boxes like these all along the south coast, soldiers scanned the horizon and targeted enemy planes and boats. This ended the practice of German shipping coming right up to the British coastline. The gun battery itself, three 6-in guns, uh, range about 14 mi. There were just five tunnels in the underground shelter.

 The main one was 50 m long and packed solid with double bunk beds on either side. The hardworking soldiers would have had just a narrow space to walk along the center. This must have been the absolute smelliest tunnel going. The fan base site was chosen because there was already a tunnel here. It dated from the First World War around 1916 going 20 m into the cliff.

 It was quicker for the army’s tunneling squad from the Royal Engineers to extend the original passage than dig a new one. If you have a tunnel that’s already started, that’s where you start. And that’s exactly what the 17cond tunning company did. They dug their way in to these and then they widened them out to become living chambers.

So, this World War II base was built quickly thanks to the pre-existence of a World War I tunnel. It was originally built into the cliff face as a shelter for the soldiers who manned these concrete sound mirror structures which date from 1916. Photographer Joe Ped Smith has been documenting these and other surviving acoustic mirrors along the south coast because his grandfather Bob Smith played a role in developing the technology.

 The two here at Dober are part of a series of 13 that are still intact and remain. When these were built originally um they were the absolute cutting edge of technology. These acoustic early warning systems predate the development of radar in the 1930s. Because they reflect and amplify distant sounds, the British Army used them to listen out for German bombers during World War I.

 So you can hear something far before you can see it on the horizon. How can you possibly try to get ground crews, you know, in position to somehow defend these structures were that? The reason that the sound mirrors and the fan bay tunnels survive in such good condition is that they were buried for nearly 50 years. In the 1970s, the British government scheme Operation Isaw cleared away unwanted wartime buildings.

Underground structures had to be sealed up so nobody can get access into them and overground structures had to be knocked flat and removed and a lot of the spoil from the overground structures was actually pushed into the underground structures to try and seal them up. But Fan Bay actually remained one of the best preserved of all of the deep shelters.

Each of these underground worlds in Dova is a testament to man’s ingenuity, reviving the disused tunnels of past eras and giving them new purposes in times of great national need. It gives Do a wholly unique feature which you cannot parallel in any other British military fortification. [Music] The Netherlands, a North European country best known for its tulips and windmills.

But few realize that it’s also the location of an extraordinary underground world, the St. Petersburg Caves. These tunnels have preserved and protected the local citizens for centuries. They created, hold on, 20,000 tunnels. Is one of the biggest cave structures of Holland. What shocking discovery was found deep inside this labyrinth? All of a sudden, they find this terrifying skull.

 And how did the World War II Dutch resistance use these tunnels and a Rembrandt masterpiece to outwit the Nazis? Once in a while, they took out the Rembrandt, picked out the radio, and sent messages to England. Around a third of the Netherlands sits below sea level. In a landscape this flat, there’s one landmark that stands out above them all. Mount St.

 Peter on the outskirts of Mastrict. It lies 200 km south of the capital Amsterdam on the border with Belgium. There’s an 18th century fort on top of the mount and beneath it lies an underground world that spans two countries. Shantel Burgger has been guiding visitors around it for 2 years. There’s something hidden underground under our feet on 30 m deep.

 Actually, hidden is one of the most biggest cave structures of Holland. The St. Petersburg cave system runs to a length of 250 km. It’s made up of two smaller networks which sit either side of the national border. The south or sonnenberg caves are in Belgium while the Nord or north system is in the Netherlands under Mastri and the fort.

 Shantel leads the way into the northern system. This tunnel is used by everyone. It’s the only entrance and exit that’s still available. The local bedrock is limestone, 2 million years old. These tunnels were first dug in the 13th century to mine it. The smart local citizens had realized there was money to be made by selling the ground beneath their feet.

And they needed building materials to build all those houses. The law was like if you owned soil on top of the mountain, you also owned everything underneath the mountain. So those farmers were as clever and came down in under here and started using chisels and saw to take out blocks out of all these limestone walls.

 And doing so they created, hold on, 20,000 tunnels by taking out one block each day. By the 1700s, the French wanted to invade the Netherlands and exploit the local wealth. By building their tunnels, the Dutch had actually created an ideal hiding place. They built a fort on top of the cave system for added protection.

 But in 1794, the French managed to make their way into the caves. They wanted to blow up the fort. If you want to see where it collapsed, have a look. This is what explosions from the French have been created in tremendous areas. Look, it’s massive. Massive. Impressive. Actually, the French army had mistakenly detonated their charges around 150 m off target, and the fort was undamaged, but they quickly learned that underground explosions can have deadly consequences.

 If you create a big explosion in a tunnel system like this, the power of the explosion divides in all the separate tunnels. So the power going up is not enough to blow away 30 m of limestone. The French soldiers were killed by the blast that funneled back at them along these passages. Can you imagine an explosion like this creating a tremendous power of air? So surely the soldiers in here must have felt the explosion back in the caves there.

 So uh must have been a tremendous powerful explosion. With the French discouraged the limestone mining continued as usual. But in the 18th century there was another seismic event in the Nord caves. Engineer and scientist V Shins has studied the mining history here for more than two decades. The working conditions were very very bad. It was dark.

 People had only little light. It was cold. It was humid. It was very dangerous to work here. So just fancy. You are here cutting blocks, sawing blocks, and all of a sudden you find this terrifying skull. The miners had dug up a monster skull nearly 2 m long and a meter wide. It was the skull of a mosasaurus, a long extinct giant sea lizard nearly 15 m long.

 The skull was more than 66 million years old. They had the bite of a T-Rex. They were able to swallow a turtle or a shark in one go. The skull was found in 1766. It must have dropped to the seabed millions of years ago. and then sediment would have buried it creating the limestone bedrock which runs under the countryside and which the Dutch miners were slowly digging up and they were frightened these people and as they were superstitious they didn’t know what to how to handle it was in the early 18th century nobody was occupied with fossils or geology or paleontology

the skull was a challenge to centuries of religious teaching. After all, the Bible makes no mention of giant creatures like this. Scientists in France demanded that the mastri skull should be confiscated for them to use as evidence of a new evolutionary story to challenge the religious order. The church was very powerful as a political power and these French uh people wanted a scientific way.

 They wanted to bring it to France in order to improve their science activities. And for that reason they brutally confiscated it here from the owner and brought it to Paris. A piece of cultural heritage of La Republic Frances. The skull is still in Paris on display in the Museum of Natural History.

 The citizens of Mastri have campaigned for the return of their Mosasaur skull, but the French are in no hurry to comply. This became this mosasaur became the very symbol of evolution throughout the world. The very fossil of an animal that proved evolution right. And it was amazing that it was found here while people were working underground just to collect blocks.

In the folklore of the St. Petersburg caves. The prehistoric mosasaur lives on. Local artist Juel Sundiker was so gripped by the cave space that in 1900 he turned the monster into a tourist attraction as part of an art installation he helped to create in the Nord caves. Four generations later, his great grandne nephew Jeanjac Spis is a guide here, proud to show off his ancestors work.

 This was uh made in 1900 and it maybe looks more like a crocodile than a mosasaur, but still it’s a very amazing piece of art here. No one had discovered a complete mossaur skeleton by 1900. So to tantalize the tourist, Sikka imagined what the body might have looked like. He added feet and legs, not realizing that mosasaurs didn’t have them, but swam with fins instead. Sundika wasn’t only a sculptor.

These chambers are full of charcoal drawings that he helped to create on areas of rock wall that was specially cut and smoothed for the purpose. He made drawings with charcoal for attracting tourists. They were mostly wealthy people. they could afford to uh travel in that [Music] time.

 And one of the main paintings was a reproduction of the famous Night Watch by Rembrandt. Rembrandt Van Rein is a renowned Dutch artist and his 1642 painting the night watch was carefully reproduced in charcoal by Sundiker on these tunnel walls. Though sadly his work has been damaged by vandals over the last 50 years.

 Jeanjac’s family still has the original copy marked up with grid lines that Zondika used to scale up his own charcoal version onto the tunnel wall. He had a replica of the night watch and he made some squares on that. And these squares he put on the wall and then he filled them. The original remance night was was oil paint and this is black and white.

 Imagine that’s done with very poor lighting. It’s really amazing. It’s such big painting and it’s very accurate done. Despite the damage caused by vandals, Sondiker’s work will soon be restored. There is another fascinating link between these tunnels and Rembrandt. During the 1940s, the original Night Watch painting was among several hundred Dutch masterpieces that were hidden in the tunnels to keep them safe from the invading Nazis.

It was simply hang on those on those iron things. Simply hang without any system. and they would layered as many as [Music] possible. Rembrandt, the most famous painter, the night watch, has one big disadvantage, its size. Nearly 4 m tall and 5 m wide, the framed nightw watch masterpiece would not fit through the doors of the vault.

 So, a radical solution was needed. They took it out of its frame. They rolled it like a simple old carpet. And he used this niche to hide the most precious painting in the world uh with a with an assurance worth at those days from 1 million gildens. And that’s the way how they rescued our precious Rembrandt.

 Hitler’s army invaded and occupied most of Europe during 1940 and Mastri was the first Dutch city to be captured in May that year. The Germans were aware of the art treasure trove, but the story goes that they were content to leave the paintings in storage. They would be safe from any Allied bombardment, and the Germans could always move the pictures to Berlin at a later date.

 The Dutch took advantage of the Germans complacency by hiding other items in the vault that were vital in the war effort against the Nazis. So can you imagine that behind the carpet rambrand something else was hidden a radio transmitter. So once in a while they took out the carpet rambrandt picked out the radio and plugged it into the electricity that was installed here.

And so they were able to send messages to England to the Allied forces whether it was good weather enough to to fly to be able to fly. Having been the first Dutch city to be captured in World War II, Mastri went on to be the first to be liberated. The US Army arrived in September 1944. The St.

 Petersburg caves have gone on to be a leading attraction, drawing thousands of tourists every year. Like diving into a total different world. From the days of protecting limestone miners from invading armies to preserving masterpieces of art, this remarkable underground world has played a vital role in the Netherlands for centuries. So, it’s one massive block of history.

Actually, that’s adorable. It’s always a mystery to walk in here. [Music] Ukraine, the second largest country in Europe after Russia. It has a population of nearly 44 million. The Black Sea Coastal Resort of Odessa is the location of one of the world’s most extraordinary tunnel networks. We are inside uh the the biggest underground system in the world.

Lying 60 m underground, these Odessa catacombs are an incredible 2,500 km long. For centuries, through all kinds of wars, they’ve protected the local people from danger. Yet, these very tunnels have also been the scene of tragedy. just to stay sane in this condition. It’s quite a sad story. All of them died eventually.

It’s such a big network that dozens of people have set foot here and never come out again. If you scream, nobody can hear you just 10 m away. On the Ukraine’s south coast as it reaches the Black Sea, the port and coastal resort of Adessa is famous for two architectural wonders. The Pmpkin steps, a flight of stairs that leads visitors from the Black Sea up to the city and the mighty catacombs.

 Locals say it’s the largest tunnel network in the world. Dr. Pavl Zagnabaroda, an army combat doctor, has been studying the catacombs for 10 years. He knows of more than a thousand entrances all over the city, but only one is open to the public. Catacombs in Rome is approximately 150 km long. in Paris like four or 500 kilometers long and here is 2,500 km long.

 So there is nothing else anywhere in the world like Adessa Catacombs. The Adessa Catacombs started life in the late 18th century as a limestone quarry. The bedrock here is 5 million years old. This limestone it has proper name. It’s called coina. Coina is a strong building material and when Odessa was founded in 1794, the citizens started mining it to build their homes.

 Little did they realize that they would end up creating a vast network that would play a major role in the future of their city. So we even have a kind of saying that Adessa was built from underground. Historic mining equipment has been placed on display in the center of the labyrinth and Ulia Pelavina has been guiding visitors around these exhibits for 2 years.

These are the tools which were found in the mines and which were used by miners in order to cut all these tunnels and to get construction material for building Odessa. The set of saws was ordered from Great Britain because mining there was much more advanced. In backbreaking conditions, the miners would chisel along the limestone seams to force out large blocks or monoliths of rock which would then be sorn up.

The process was very very hard and very time consuming. Keep in mind that in one shift, which could be about 10 or 12 hours, a group of six miners, six really strong men, could only make a corridor 1 and half meter long. By the 1870s, Adessa had grown to a population of nearly 165,000 people. Demand was high for the limestone as the city kept growing.

 But after the Russian Revolution in 1917, mining was stopped. New industries like metal working and machine building took priority in the new Soviet Union. But the catacombs found new uses. When the big mining had stopped, people started using the catacombs for all sorts of purposes. So then all sorts of criminals have been hiding here like smugglers and thieves and pickpockets.

 They were hiding here from the police. They hid here their belongings, their stuff, guns, money. The tunnels beneath Adessa were to play their most vital role for locals during World War II. In 1941, Nazi Germany decided to invade the Soviet Union. Hitler’s plan was cenamed Operation Barbarasa.

 Odessa was a key target and fell to German and Romanian forces in October 1941 after a 4month siege. 82,000 citizens of Adessa were killed and another 78,000 taken prisoner as slave labor for Germany. But a local resistance movement called the partisans attacked the Axis invaders wherever possible. One group of of partisans they during the second world war they stayed in this part of of catacombs.

There were around 45 such resistance groups totaling 6,000 fighters. Their graffiti lines the tunnels and their posters are on display calling for their property to be reclaimed from the fascist invaders. The Axis soldiers sealed hundreds of tunnel exits hoping to kill the partisans. Pavl knows the story of one group of 13 men who were trapped.

They couldn’t go out. All the exits were blocked. So just to stay sane in this condition, it it is it is a very big deal. It’s quite a sad story. All of them died eventually. The story goes that months of living and surviving underground drove the partisans mad, causing them all to kill each other. Only one survived, Alexander Glucenko.

He spent another one year on his own underground and then he managed to get to his home just nearby and his wife hid him in the catacombs under their house for another year. So, so he spent more than three years on the ground. So this guy who stayed here for three years and and remains saying he’s really a hero to me.

 Apparently, Glenko’s survival was inconvenient. After the war, the Soviets wanted to suppress information about the partisans. Instead of rewarding him, they silenced him. Unfortunately to him, when the Soviet army freed the city in a couple of days, they just shot him dead. They didn’t want anybody knew about the truth. What happened to heroes of the war? [Music] Some partisan groups had set booby traps to kill any Axis soldiers who came looking for them.

 These bomb traps needed removing after the war to make the catacombs safe. This was a time of the liberation of our city from the occupational Romanian and German forces during the Second World War. and military engineers, they had to kind of clear up the place and when they finished checking, they had to leave this sort of messages for the rest of the people that this place is safe.

 So it says checked, no mines, so it’s a safe place. Now you can see here the date since the 14th of April of 1944. After World War II, the Cold War era lasted from the 1950s to the late 1980s. The Soviets converted large areas of the catacombs into atomic bunkers, each of which could house several thousand people in the event of a nuclear attack.

It was a constant fear of possible nuclear war which could start at any time. They built around 80 anti-uclear bomb shelters in a desert. You can see how builders changed the catacombs for their purposes. They didn’t have to dig anything. They already had a big space underground, but they needed to fortify here some walls and ceilings.

 As part of the natural geology, these rock layers are riddled with cracks. It would have been impossible for engineers to seal them up against the shock wave of a nuclear blast. So, they came up with an ingenious solution to keep the shelters pressurized. It was supposed to withstand very big pressure from outside.

 So what they did they used their air pump system and they created positive over pressure inside of the shelter. This pipe the big pipe for air it’s the same system works in submarines in planes. So this way you create kind of air tightness so nothing can get through from outside. So that is why air filtration and air ventilation system was so essential.

Power for high air pressure and other life support systems came from vast engines. They were lowered down vertical shafts which were then sealed. This place is really big and it was designed for more than 5,000 people. So they needed a lot of electricity, a lot of power to keep this place in proper condition.

When the Cold War ended in the late 1980s and the military powers withdrew, the bunkers were abandoned and their contents looted for scrap. Only onethird of the catacombs has been mapped. 2,500 km of tunnels are impossible to monitor. A handful of urban explorers ventures down into these catacombs without realizing the dangers.

People just come down here. They just get lost. It’s really easy to get lost here. At least one person goes missing in the catacombs each year. Fortunately, the city has dozens of amateur explorers who team up to find them. It’s very difficult for rescue teams to find those lost ones because this rock has a very interesting quality.

 It can absorb all the noises. So if you scream, if you shout, whatever you can do here, nobody can hear you just 10 m away. Despite the dangers, the local people think these tunnels have special qualities. For instance, at the Shustaf Cognac factory, which sits above ground, they opened a museum in the catacombs in 2013 to commemorate 150 years of maturing their prize cognac down there.

There is no other place in Odessa like this because they use the underground spaces to store their cognacs. The Shustaf cognac was always placed in the catacombs because the people believed the cockina limestone imbued it with a mystical quality. The process of maturation uh takes place underground.

 The cognac itself, it acquires the energy of our odessa limestone bottle is this cognac is 128 years old. It’s the oldest cognac in the territory of Ukraine. One of the cognacs is named after the catacombs and museum visitors drink in the ambiance at this popular underground venue. They organize parties here. They have concerts.

 They have an art gallery. They have underground cinema. So unique place [Music] from the coina mine which provided the limestone to build the city to the bomb shelters which would have kept the citizens safe. These catacombs have played a vital role in this city’s history. Catacombs is a big thing for for Adessa and for our people.

 So we have buildings, safe houses above and we have safe places underneath. We are happy to share this experience with all those who come to our city, who visit our catacombs. [Music] [Music]