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00:00Beneath the surface of our planet lies a vast realm.
00:06Look at this.
00:07Countless caves, hidden tunnels, and submerged rivers.
00:15Now, on a quest for knowledge, expert cavers are descending into the uncharted abyss.
00:22Descending!
00:24Where extraordinary discoveries lie waiting.
00:27Looks like a very kind of violent act.
00:30Armed with the latest scanning technology to penetrate the darkness.
00:35Here it is.
00:37But look at all the detail. That is amazing.
00:40And turn data into accurate 3D images.
00:46This time, in the far east, a monumental cave chamber.
00:53How?
00:54Pushes Underworld to new extremes.
01:02Can the team descend a terrifying drop into a cavern taller than the Eiffel Tower?
01:08All of my worst fears can be found in caves like this.
01:11Can Underworld bring light to the biggest cavern on Earth?
01:16Okay, okay. Very good. Very good.
01:18And together, with groundbreaking scanning technology, uncover the secret to how clouds are formed deep within a mountain.
01:27I've never seen anything like it anywhere.
01:29I've never seen anything like it anywhere.
01:30It's unique.
01:31It's the most populous country on Earth.
01:45Where ancient tradition and the modern world meet.
01:59China.
02:00But beyond its frenetic cities, lies one of the largest limestone landscapes on the planet.
02:10With spectacular stone forests.
02:14The world's deepest sinkholes.
02:17And a tantalizing underworld.
02:21China.
02:22China is the ultimate caving location.
02:28With the greatest concentration of undocumented caves anywhere on Earth.
02:34And as cave explorers chart this new frontier, they're discovering some of the largest caverns on Earth.
02:47Known as Mountain Halls.
02:57The Underworld team is heading to one of these mega caves.
03:01It lies in the Chuengkodong cave system in southwest China, near the city of Chongqing.
03:18The team's mission is to chart a strange phenomenon inside the cave chamber.
03:24Locals report that this cavern creates its own clouds.
03:31They hope to see this vast space for themselves.
03:34And discover the secret of how weather is made underground.
03:40There are lower entrances into the Mountain Hall.
03:44But to explore it from top to bottom.
03:46The team must take this route.
03:48Leading to the cavern's highest accessible point.
03:51And work their way down.
03:53Which means starting at this dark and uninviting entrance.
03:57Leading the expedition is expert caver Liu Jia.
04:10One of China's foremost underground explorers.
04:14With over 17 years of experience, she's been nominated as one of China's top ten cavers.
04:19We plan to enter from here.
04:22It is called Zhai Cave.
04:24And then from here, you have to walk for over two hours.
04:28Accompanying her are a trusted team of cavers.
04:34Each with crucial skills vital to the mission.
04:38We want to start from Zhai Cave and measure the temperature and humidity at each point.
04:46To see how clouds and mists are formed.
04:51The team is entering the Chuen Kodong cave system through Zhai Cave.
04:57From here, they'll negotiate the twisting passage for nearly half a mile in order to reach the giant chamber.
05:05The entire mission will take at least 24 hours.
05:11Once inside the mountain hall, it will be difficult to appreciate its full size with only caving headlamps.
05:20The cavern was first lit and photographed successfully in 2012.
05:25It made headlines as the poetically named Cloud Ladder Hall.
05:34But what's creating the clouds inside Cloud Ladder Hall is a mystery.
05:40One of the few cavers to have witnessed it is 3D scanning expert Ru Walters.
05:46Clouds in caves are very rare.
05:49I can only think of places in China where I've ever seen them.
05:52I've only seen two places where the cloud has become so thick that you actually have trouble navigating.
05:58One in Cloud Ladder Hall, I couldn't actually see my feet on the ground if the cloud was that thick.
06:03Little is known about how clouds form underground.
06:06Outside in the atmosphere, clouds are often created when the sun's heat evaporates moisture from the earth and sea.
06:16As the warm air rises, it cools and the water condenses, forming clouds of tiny water droplets.
06:24These droplets can merge, eventually becoming so heavy they fall as rain.
06:29The question is, without the sun's heat driving the process, how can a cloud form deep underground?
06:43Liogia wants to reach the vast cavern to discover its secret.
06:48The team's plan is to take temperature and humidity readings at various points in the cave system.
07:04This will help them analyze differences in atmosphere and reveal possibilities.
07:18the possible clues to how cloud is forming.
07:22And to help them navigate, they brought along a secret weapon.
07:27A scan of the cavern created by Rue Walters in 2015.
07:39Wow, it's really hard.
07:42But the scan doesn't include their route in.
07:45It will be an arduous journey navigating into the heart of the mountain before they can even reach Cloud Ladder Hall.
07:56So why is China home to such enormous cave chambers?
08:01It's all due to the sheer scale of its unique geology and tropical weather.
08:08Well, China's limestone landscape is interesting mainly because it's so huge.
08:15It is vast.
08:16You get these amazing rivers ambling through this landscape of paddy fields and these big towers of limestone coming up. Extraordinary.
08:25The scale of them is not unlike anywhere else.
08:27Karst landscapes stretch right across China.
08:33The greatest concentration is found in the south, covering an area the size of Spain.
08:40It's here that some of the world's largest caverns can be found.
08:44In places, China's karst is thousands of feet thick, enough for huge mountain halls to develop.
08:55A process only possible with something else that China has in abundance.
09:02In abundance.
09:06Water.
09:08Over hundreds of thousands of years, tropical monsoons have deluged these mountains, forming underground rivers that erode the rock.
09:18And as it trickles through the soil, it gets acidic.
09:21And that dissolves away the calcium carbonate.
09:24And as it goes, it just kind of opens up and enlarges natural fissures, natural cavities.
09:31And eventually, of course, those turn into caves.
09:34As those caves grow, they transform into mountain halls thanks to the depth of the limestone and the volume of the rivers.
09:44As rivers flow through mountains, they erode the floor beneath them.
09:48As the water level drops, air spaces form in the cave above.
09:56Without water to stabilize the cave structure, the roof can collapse.
10:03Eventually, creating large cave chambers, which grow bigger over time.
10:08But exactly how big China's mountain halls are is something cavers and scientists are trying to discover.
10:22Rue's scan of Cloud Ladder Hall was part of an international mission to find out once and for all how China's large caverns compare to others around the world.
10:32We really didn't appreciate just how high many of these caves are.
10:39The scanning process that we have works with lasers and works in the dark.
10:43So we don't need light to be able to see what's going on.
10:46And this has enabled us to see the true dimensions of the place.
10:50We're heading right up into the roof.
10:52And what the scans revealed was a scale of cave rarely seen before.
10:57Hong Mei Gui Chamber.
11:02It has a height of almost 740 feet.
11:06And a ground surface area bigger than 11 US football fields.
11:13But it's only the sixth largest mountain hall recorded.
11:17A cavern called Yongguang, or Funnel of Light, is the fourth largest.
11:22While its floor area is smaller, it's over 300 feet taller than Hong Mei Gui.
11:30But the largest in China, and also in the world, is the mighty Meow Room.
11:37It is over 2,700 feet long.
11:41And is large enough for a Boeing 747 to pass clean through.
11:46Experts are still arguing over the criteria that define the world's largest caves.
11:53But it's clear to see that China's mountain halls are massive.
11:58These caves are humongously vast.
12:02These are on a scale that are just mind-blowing compared to anything that we've seen before.
12:06And to this day, it's really, we don't understand precise mechanisms of how these places actually stay up and are incredibly stable.
12:19The Underworld team is deep underground, heading towards another gigantic mountain hall.
12:25But to get there, they are having to negotiate their way through the long entry passage.
12:34The route is getting narrower.
12:38And steeper.
12:42Good teamwork is essential to get themselves and their kit through these tight squeezes.
13:00It's exhausting and difficult caving.
13:17And soon, they meet their biggest challenge yet.
13:22A hazardous climb up a steep vertical shaft.
13:25It's the only way to get close to the top of Cloud Ladder Hall.
13:34But straight away, there's a problem.
13:37A boulder is blocking their exit at the top.
13:42In ever-changing karst caves, loose rocks like this are simply part of the process of cave formation.
13:52But to the cavers, they're an obstacle that must be removed if they want to carry on.
13:59It's a risky maneuver.
14:02The rock could crash down on the team below at any moment.
14:05Let's go.
14:06Let's go.
14:07Let's go.
14:08Come on.
14:09Go to where?
14:10From where?
14:11You're away.
14:12You're away.
14:13Let's go.
14:14Let's go.
14:15Let's go.
14:16Make sure the people are all in the left.
14:18Make sure you're on the left.
14:19From there.
14:20Take one.
14:21Okay, if I got wrong.
14:24Look, okay.
14:25OK, no, it's not good.
14:27Let's go.
14:28Come on one other way.
14:29One one is on this.
14:30Don't help, please.
14:31Good.
14:32One more is on this.
14:33Despite the danger, this sort of challenge is exactly what attracts the team to caving.
14:40Danger is probably one of the attractions of cave exploration.
14:45Also turbulent underground rivers, rugged cave exploration routes, descending high cliffs,
14:52they're all part of the challenge.
14:56In fact, danger is the essence of cave exploration.
15:02But their problems aren't over yet.
15:05As they climb up, the humidity is intensifying.
15:11And the damp conditions are making the rock surface extremely treacherous.
15:23In these conditions, it takes over an hour to get the whole team and the gear up the narrow shaft.
15:32Karst is a dynamic environment.
15:39Just as it creates tall shafts and enormous chambers underground,
15:44it also makes stunning vertical rock formations above ground.
15:50Landscapes of dramatic pillars and stone forests,
15:55which in ancient times inspired names like elephant rock.
16:02These formations provide a close encounter with this vertical world,
16:07thanks to a new generation of adventurer.
16:10Over 360 miles from Cloud Ladder Hall in Yangshuo County, Guangxi,
16:17lies a formidable and fragile rock formation.
16:21A monumental limestone arch.
16:26Moon Hill allows a close-up view of this unique vertical world.
16:39But only for the most daring.
16:45Leo Yangbang, also known as a bond, has climbed mountains all over the world.
16:55But he believes China's karst is the best.
16:58Why does Yangshuo attract so many climbers?
17:03Because the limestone here has the richest rock texture in the world.
17:13You can pinch, clutch and make finger holes, do everything with the limestone.
17:21These walls are the remnants of underground caverns,
17:25and like caving, they test his abilities and build his strength.
17:36Normally cliffs and mountains have a vertical faces, but this is arch-shaped.
17:41It puts great demand on your core strength.
17:44If you let your feet swing back and forth,
17:47your forearms get weaker and you could fall.
17:51For handholds, he relies on stalactites formed by dripping water.
17:56But some are thin and fragile and could easily break.
18:00Yet it's the dangers of climbing in an unstable environment
18:04which makes it all the more exhilarating.
18:11The feeling of holding on to stalactites
18:14and the possibility of falling off is a thrill.
18:19People come here for their challenge.
18:24A bond uses the water-eroded pits and ledges in the limestone as footholds,
18:29and at the top for a brief moment of relaxation.
18:34Such an intimate view of this vertical world of karst is reserved only for those with stamina and strong nerves.
18:47It's these qualities that are also vital to Liu Jia and her team of committed cavers.
19:00They are getting nearer to the focus of their mission, Cloud Ladder Hall.
19:06To discover how the cavern generates its own clouds,
19:10they'll need to take temperature and humidity readings at different points within the main chamber.
19:16But to reach the bottom, they will have to complete a nerve-wracking rope descent.
19:21Long abseils like this come with dangers, so it's vital to prepare.
19:33Liu Jia uses a short cliff on the route as an opportunity to train the team.
19:38Abseil techniques vary, but to control their descent, the team is using a device called a Stop Descender.
19:55It allows a caver to stop and start the descent during an abseil or rappel.
20:02To move down, the caver presses a lever and the rope passes through the device.
20:08When they want to stop, they release the lever, locking the device by wrapping the rope around it.
20:15On a rappel of this length, they must take breaks, not only to avoid getting tired hands,
20:21but also to prevent friction making the descender too hot to touch.
20:28I know from bitter experience that these devices can actually get just ludicrously hot.
20:34And if you don't stop regularly, that device gets too hot to handle and you can't control it.
20:39And that, of course, is dangerous.
20:42And with over 800 feet of heavy rope to deal with,
20:46it's important that the team practices techniques to help them handle it.
20:53As each caver will only have themselves to rely on during the descent,
21:05the whole team needs to know what to do in a crisis before they reach Cloud Ladder Hall.
21:10The team presses on, and before long, the cave opens up.
21:29After more than eight hours of grueling caving, they've reached what they came here to see.
21:34Through a small entrance lies a bigger chamber full of mist.
21:43This is Cloud Ladder Hall.
21:48The cavern is so big that when the team throws a stone down into it,
21:56it takes several seconds for the echo to come back.
21:59Because of the echo, we can tell that it's a very big cave.
22:06And in the beam of the flashlight,
22:09the dense cloud creates a stunning and unusual phenomenon
22:13not normally encountered underground.
22:15You can see it.
22:16It's like a light.
22:17It's like a light.
22:18It's like a light.
22:19It's like a light.
22:20It's like a light.
22:21It's like a light.
22:22It's like a light.
22:23It's like a light.
22:24It's like a light.
22:26Known as the light of Buddha, or a broken specter.
22:31This optical illusion is caused when a backlit object is projected onto thick cloud.
22:37Light is distorted through the water droplets,
22:39and the figure is magnified to giant Buddha-like proportions.
22:56The team takes their first readings inside Cloud Ladder Hall
23:00to try to understand how it creates its own cloud.
23:03The results from their position near the top of the cavern
23:08show a temperature of around 63 degrees Fahrenheit.
23:14But it's extremely humid compared to the cave system's entrance.
23:1997% or higher.
23:21Not a surprising result given that the cloud here is so thick.
23:24Some of the water vapor is already condensing onto the walls in droplets.
23:33But how will the atmospheric readings compare at the bottom of the cavern?
23:37To orientate themselves for the next part of the mission,
23:42the team turns to the water vapor into the water.
23:45The water vapor is already condensing onto the walls in droplets.
23:48But how will the atmospheric readings compare at the bottom of the cavern?
23:51The time is пока, yes.
23:53While the water vapor is closed to the water vapor…
23:54…to check out the water vapor.
23:55We are able to work with that.
23:56This is the area.
23:57This is the metro area at the top of the tunnel.
23:59And we might have to wait for that one.
24:01The scan can help them plan their way through the mist and darkness
24:05during their long abseil down into Cloud Ladder Hall.
24:08It looks like a view of the water vapor.
24:12The 3D data shows the team what they cannot see.
24:17It reveals an entire virtual underworld,
24:27a vast natural chamber.
24:32From the team's entry point at the top of the cavern,
24:36the scan reveals the route of their perilous abseil down,
24:42all the way to the cavern's base,
24:44where more obstacles lie in wait.
24:49A field of huge boulders.
24:53It's a terrifying but awe-inspiring sight into what lies ahead.
25:04Through the scan, I can see all the conditions in the cave.
25:11It vividly reflects the environment inside, in detail.
25:22To prepare for the descent, two anchor points are drilled into the rock.
25:26If one fails, the other will act as a backup.
25:33With each team member abseiling one by one,
25:39if there are any problems, no one can help them.
25:42But as experienced cavers, they've all learned to overcome many of their primal fears.
25:49All of my worst fears can be found in caves like this, vertical cliffs, dark confined spaces.
25:57There are also a lot of insects.
25:59All the things I fear are there.
26:01For me, I like to combat poison with poison.
26:04By facing my fears, I overcome them.
26:07As the first caver, Lu Chongqing, secures the top descender and begins to lower himself down,
26:14there's the thrill of finally entering the colossal chamber.
26:19I will see you later.
26:35But before long, there's an urgent issue.
27:01The long rope has become knotted.
27:04It cannot pass through the stop descender, leaving Chongqing stranded.
27:16This could jeopardize the whole mission for Liu Jia.
27:28It's vital Chongqing unknots the rope, or the rest of the team cannot descend.
27:34Untangling 700 feet of heavy rope whilst dangling in midair is an exhausting task.
27:49But thanks to his safety training, Chongqing stays calm and methodical.
27:54After several agonizing minutes, the rope is finally free and Chongqing can continue his descent.
28:05One by one, the rest of the team follows suit, until all five of them arrive safely on the chamber floor.
28:26In the gloom of the vast cavern, they pick their way across a field of boulders using only flashlights to look for a place to set up camp.
28:41It's been an exhausting journey, much longer than anticipated, around 15 hours of challenging caving.
29:00As it's now past 1am, their priority is to rest and eat.
29:08Though with limited options, it's impossible to please everyone.
29:15If they could only see their surroundings, this would be one of the most spectacular places on earth to spend the night.
29:30Visualizing these vast wonders is a problem all cavers face in China's mountain halls, especially when trying to capture them on film.
29:49These huge caves are a real minefield for the photographer.
29:55So you need considerable amounts of light in the right places.
29:59And you usually find that one photograph in these places can take 3 or 4 days of setting up and a very, very long day to take.
30:08Around 300 miles from Cloud Ladder Hall in Guizhou Province, lies another great mountain hall,
30:15which poses a particular challenge to photographers and filmmakers.
30:24First surveyed by cavers in 1989, the Meow Room is named after the ethnic group that lives in this region.
30:33It's the largest mountain hall in the world, the one that's big enough for a passenger jet.
30:38But at over half a mile in length, its elongated shape makes it especially difficult to capture on camera.
30:47Now, another Underworld team is on a mission to record more of it on video than ever before.
30:54And they have a plan for lighting the gigantic chamber.
30:57We will be using the best flashlights in China, 20 of them, with 18,000 lumens each.
31:13Not only has the team brought along the best technology available, they've also got manpower.
31:19An unprecedented team of more than a dozen cavers.
31:25Of course, we know it's going to be hard because it's really big and dark.
31:33But we have a lot of help from many friends.
31:37To find the best place to capture as much of the cavern as possible, the team heads to its most open section known as Great Camel Hall.
31:58They must navigate their way through many obstacles in the pitch black.
32:02Their headlamps illuminating only tantalizing glimpses of the cave floor.
32:14You'd never know they were inside the biggest cave chamber on earth.
32:25Finally, the team spots a known landmark, which tells them they've reached their destination.
32:33We have now arrived at Great Camel Hall.
32:37This is in the mail room.
32:39We are at the very center where the stalagmite can be found.
32:43Using radios, they begin to coordinate themselves around the cavern.
32:54Spacing themselves out, each person carries one of the powerful flashlights.
32:59Once the camera is positioned as far back as possible, it's time to begin.
33:02Once the camera is positioned as far back as possible, it's time to begin.
33:07How long is it?
33:09It's time to go to the back as possible.
33:10Now, let's go to the back as possible.
33:11Now, let's go to the back as possible.
33:12Now, let's go to the back as possible.
33:13Three, two, one, open the door.
33:17As the mountain hall's unique features come into view, it's possible to begin to get some sense of its vast scale.
33:28Okay, okay.
33:29Very good.
33:30Very good.
33:31Very good.
33:32Very good.
33:33Very good.
33:34Very good.
33:36But even with all 20 powerful flashlights, only a fraction of the central part of the lung chamber is lit up.
33:43To light up the whole of the meow room fully would likely require hundreds more cavers and lights.
33:56And this is not something the team in Cloud Ladder Hall could contemplate either.
34:06Even though it's the next morning, the cavern is still just as dark as the night before.
34:13For them to get a sense of scale, they need Rue's scan.
34:18The scan can show details flashlights alone could never reveal, and clearly illustrates the cavern's massive size.
34:31With a volume of over 20 million cubic feet, Cloud Ladder Hall is the third largest mountain hall on Earth.
34:41And with a height of over 1,100 feet, it's also the tallest, even higher than the Eiffel Tower.
34:53Could it be that the cave's great size contributes to cloud formation inside?
34:59To find out, the Underworld team takes another temperature and humidity reading from the bottom of Cloud Ladder Hall.
35:19The readings reveal an important clue, a temperature gradient.
35:24It's warmer at the bottom of the cavern than it is at the top, by almost seven degrees.
35:32This suggests the clouds in here could be produced in a similar way to how they're formed outside.
35:39With warm air rising from below and condensing as it meets cold air higher up.
35:45And it's the cave's great size and height which could allow this to happen.
35:51The warm air has a unique opportunity to actually rise quite dramatically, you know, over 300 meters.
35:57And that therefore, by the very nature of the way these things work, means that that air will cool.
36:02And that water will, you know, have the potential to form water droplets and then form clouds.
36:08But you'd expect air inside a cave to be cold.
36:12So where is the warm air coming from?
36:15It's not long before Xiao Hong discovers a clue.
36:30There is air flowing at the base of the cavern, something which Ru Walters also experienced when he scanned Cloud Ladder Hall.
36:37The wind is all over the place. It's coming from one direction, one second.
36:42And, you know, it's very, very gusty and quite ferocious.
36:46It's certainly the windiest place I've ever been in a cave.
36:49Warmer and more humid air is being drawn inside Cloud Ladder Hall at its base.
36:55The team knows that the cavern has lower entrances, but need to explore them further to understand what might be drawing in the air with such force.
37:10But before long, they come across something else intriguing.
37:15Three mysterious crater-like holes in the ground.
37:20Each around six feet across.
37:23Their matching size and shape suggests they were man-made.
37:29In fact, they're evidence of China's historic relationship with the underworld.
37:34People have been drawn to China's caves for millennia.
37:43While in the West, caves were often associated with terrifying evil.
37:49In the East, they were believed to hold special properties.
37:55And even the secret to eternal life.
37:58According to the ancient religions of Taoism and Buddhism, caves were spiritual places.
38:06The Taoists believing they were filled with chi, or life energy.
38:11There's a long history in China of using caves, and this is recorded in historical documents.
38:16They believed caves act as portals and passages to ascend to heaven.
38:22Believers would pray, meditate, and even live in caves.
38:28With many also believing that substances found inside had healing properties.
38:36Bat droppings, mineral deposits, and earth from cave floors were all collected and used in traditional Chinese medicine.
38:45Taoist monks also searched caves for a physical solution to immortality through a branch of experimental science called alchemy.
38:58The whole point of alchemy is that you're looking for elixirs of life and substances that will give you great longevity.
39:05The alchemists of ancient China purified and combined ingredients, including those found in caves, to create potions.
39:15It was believed that these magical concoctions could turn lead into gold, cure crippling diseases, and even outsmart death itself.
39:27But if these pits were dug by alchemists, what were they mining?
39:37Leo Jia has an idea.
39:40These are actually pits that people use to boil nitrate.
39:46Because nitrate is one of the materials used in the manufacture of gunpowder.
39:51Gunpowder was one of alchemy's unexpected inventions.
39:59Stable, yet highly flammable.
40:04It was accidentally created during the 9th century Tang dynasty.
40:11In their quest for life-extending elixirs, alchemists combined precise amounts of sulfur, charcoal,
40:18and the main ingredient, saltpetre, or potassium nitrate,
40:24a naturally occurring mineral found in caves.
40:29And what they produced revolutionized warfare.
40:33Over the next four centuries, gunpowder allowed the Chinese to develop sophisticated weaponry,
40:39such as flaming arrows, bombs, rockets, and even cannons.
40:45Putting them streaks ahead of Europe.
40:50The demand for this black powder created a booming saltpetre industry.
40:56And people would be prepared to journey deep underground to reach it.
41:05These early miners had a unique way of finding the nitrate.
41:08In order to locate it, they would first taste the earth.
41:12If it was salty, then it was likely to contain nitrates.
41:14You would want to get the item in theاء and that's it.
41:15But the first to reflect the earth.
41:16If it was salty, then it was likely to contain nitrates.
41:17But the nitrate pits could also give the team a vital clue.
41:18But the nitrate pits could also give the team a vital clue
41:44to how warm air is being drawn into the cavern
41:49through an essential component of the mining process.
42:03Without a large water supply, nitrates couldn't have been extracted.
42:14But today, the cave around the team is dry.
42:21There is no visible running water.
42:24So where was it coming from?
42:27As the team explores further around the cavern's base,
42:31they come to one of its lower entrances
42:35and make a discovery.
42:40Telltale green vegetation suggests water is not far away.
42:46This is something that Ru Walters also heard rumors of.
42:51During the rainy season, we know from, you know,
42:54talking to the local people,
42:55that a sizable river flows into that cave.
42:58You know, it would be like any mountain stream
43:00you'd see anywhere in the world just thundering into this cave.
43:03Today, the water level is low.
43:08But after heavy rains,
43:10eyewitnesses report that the river flows through a large passage,
43:14entering the base of the cavern from the north
43:17and leaving to the south.
43:21It's this flow that could help draw warm air into Cloud Ladder Hall.
43:26You know, we know from other caves
43:28that rivers flowing through caves draw in air
43:31and create turbulence and create wind.
43:33And if it's humid outside,
43:34that means that warm, humid air
43:36is going to be drawn into the cave.
43:39But as the river is seasonal
43:41and isn't always flowing each time a cloud forms,
43:44it may not be the only factor.
43:47The answer may lie in the configuration of passages
43:50that make up the entire cave system around Cloud Ladder Hall.
43:54The cave system has multiple caves connecting to each other.
43:59And it has multiple exits.
44:03Adding the other passages onto the map reveals the whole
44:15Chuen Kodong cave system.
44:19Four passages radiate out from the cavern's base in all directions.
44:27Measuring over nine miles in total with several different entrances,
44:33this is a vast network with Cloud Ladder Hall at its heart.
44:38It's the complexity of Chuen Kodong that could be a vital factor in the clouds' formation.
44:49All these different passages, they come from different places,
44:53different conditions at any moment in time on the outside.
44:57So all of that air being dragged into the cave at different,
45:01slightly different pressures are probably what's leading to the incredibly turbulent air
45:06that you find within Cloud Ladder Hall itself.
45:13With the Chuen Kodong cave system revealed in all its glory,
45:17we can see how Cloud Ladder Hall's weather system likely works.
45:24Sitting at the center of the Chuen Kodong cave system
45:27is the mighty main chamber.
45:30The multiple passages at the cavern's base provide many routes in for the warm air.
45:35often also drawn in by the flow of the river.
45:41As this warm air converges inside Cloud Ladder Hall,
45:45the great height of the cave allows it to rise and cool naturally,
45:50as it meets the sheer volume of cold air contained within.
45:54And as it cools, the vapor condenses into water droplets,
46:02creating the cave's massive and enigmatic cloud.
46:07Cloud Ladder Hall has exactly the right features to allow it to create and harness a cloud.
46:12A unique set of conditions that aren't found in every large cave chamber.
46:33Cloud Ladder Hall has its own underground microclimate.
46:36A weather system just like that in the outside world.
46:42And with no way out from the top except the single passage the team entered by,
46:49clouds get trapped inside.
46:52I've never seen anything like it anywhere.
46:54On the surface, I can't compare it to anything.
46:56It's unique.
46:58As the team starts to make their way out of one of the lower entrances,
47:06it's clear just how lucky they've been.
47:09For there's a real possibility that both Cloud Ladder Hall and its cloud may not be here in the future.
47:16All mountain halls will one day disappear.
47:26Leaving behind what are known as Qian Kung, or sky holes.
47:35These formations are the final stage in the evolution of mountain halls.
47:39As more and more limestone falls from the unstable chamber ceiling,
47:47eventually the entire roof will start to cave in, creating a sky hole.
47:54Sky holes commonly known as sink holes can form all over the world.
47:59But the karst regions of China boast the biggest ever discovered.
48:13China's Da Shui Tiang Kung is one of the largest sinkholes in the world.
48:19At around 2,000 feet wide and over 2,000 feet deep.
48:23And one day, Cloud Ladder Hall could end up just like it.
48:31Cloud Ladder Hall is, of course, doomed.
48:34Gradually, the erosion processes are eating away on the inside of the cave,
48:40into the mountain, and it's reaching further and further and further and further up.
48:44And eventually, it will break out onto the surface.
48:48This process is already underway at another of China's mountain halls.
48:53A funnel of light.
48:54Here, a small hole in the roof allows the mist inside to escape.
49:00With impressive force.
49:08Just when this might also happen at Cloud Ladder Hall is uncertain.
49:14It could happen tomorrow that they finally break through.
49:18There's an earthquake and it breaks through.
49:19It could happen in 100,000 years. There's no way of telling.
49:23It's a bitter irony that with southern China's constantly evolving karst landscape,
49:30the very forces which created its magical mountain halls could one day destroy them.
49:35I feel very lucky to be able to see it now.
49:43I hope to see it a couple more times.
49:47One day, it will collapse and become a sky hole.
49:54So when I stand inside, I feel like I'm a witness to history.
49:58With their exploration of Cloud Ladder Hall now at an end, Underworld has finally completed its mission in China.
50:12It's been a challenging and difficult journey with many unexpected hurdles to overcome.
50:21But the mystery of China's most enigmatic cave phenomenon has been unraveled.
50:29And for the first time, the possible secret to what's creating the cloud inside Cloud Ladder Hall has been visualized in stunning 3D.
50:40But there is still so much work to be done to fully understand the science of Cloud Ladder Hall.
50:52We're only just beginning to understand the mechanisms that are going on in Cloud Ladder Hall and indeed all the other big chambers around the world.
51:01We've got some ideas about how they're formed.
51:03And we've got some ideas about how the clouds are forming.
51:06But a great deal more work has to be done before we really know for sure.
51:12More scientific missions are needed to further understand this great wonder of the Underworld.
51:18Before it's too late.
51:19The Underworld.
51:20Before it's too late.
51:48Transcription by CastingWords
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