The Discovery Of The World's Biggest Cave

Deep in the jungle of central Vietnam, there's a wonder of the world. Wow, that is amazing. A lost universe that hides a unique ecosystem. It doesn't happen anywhere else on this planet. I think that's just about the holy grail for a biologist. And a team of scientists and explorers with a mission. This is an amazing cave. Look at that. Absolutely staggering. To discover what makes this the world's biggest cave. Vietnam's Mountain River Cave, or Han Son Dong, is uncharted territory. More people have trodden on the moon than have entered this giant cave system. This team of scientists and cavers are about to embark on a journey of discovery to see if this cave is the largest in the world. This is an absolutely amazing cave. It's huge. We're just in the entrance now. It's going to get so much bigger. That's amazing. Absolutely wonderful. This is definitely a once-in-a-lifetime opportunity. I'm so excited about being here. Practically the first person to set foot into the cave. It's a wonderful opportunity. In 2009, caver Jonathan Sims was one of the first humans ever to enter here. But deep into the long cave passage, he was stopped short by a forbidding rock face. Now he's back with caving expert and expedition leader Howard Limbert. He has the right climbing equipment to climb the wall and a commitment to crack the secrets of this cave and claim a world record. The expedition begins with a dangerous 100-meter abseil into the first vast passage. British caver Howard Limbert has led 12 expeditions to Vietnam's limestone caves. This year's expedition into Mountain River Cave is especially dangerous because it's so remote. Anybody can fall, twist their ankle, break their ankle, that could happen. And if that happens, you know, then we've got a rescue on our hands. German zoologist Danette Boettcher is impatient to get down into the darkness. She suspects a unique underground world will have evolved in this huge cave. Scientific opportunity and a caving opportunity as well at the same time. It's fabulous. Darrell Granger is here to unravel the mysteries of Mountain River Cave. He's a world expert in cave geology. If this is the world's biggest cave, he wants to know how and why it grew to such record-breaking size. It's like no other cave I've ever been to. Southeast Asia is famous for its 450-million-year-old limestone geology. Many large caves litter Vietnam. But here, within the Ka Bang National Park, Mountain River Cave is certainly a giant among caves. In the 2009 expedition, Limbert and his team were only able to fully explore the first sections of the cave. But they found areas they estimated to be 150 meters wide. And 200 meters high. That's enough room to fit London's Big Ben in twice over. Deer Cave in Sarawak, Malaysia currently holds the title world's biggest cave. From what they saw last time, Limbert's team is convinced this cave may be bigger. Get the map of the cave in some sort of order. But if they're to prove it and claim the world record, they need hard evidence. All this is unmeasured. That's why we've got to survey to find, you know, just how big it is. Whether it is the biggest cave in the world, we've got to do a serious survey. This passage here for surveying, it was really, really hard work. It was a muddy... Jonathan Sims is one of the four cavers who drew up this rough original survey. What we really need to do next is get over this wall. He knows the wall that defeated him back in 2009 is their biggest challenge. It's a dangerous obstacle to overcome. Made of loose calcite and mud, it stands at least 14 metres high. And to climb it will take all their combined expertise and effort. We just weren't expecting anything like that. Well, as well as that. Sims is determined this trip will be a success. He won't fail twice. If they can climb this forbidding wall, he's convinced he'll discover the cave continues and the world title will be theirs. They estimate the wall is around five kilometres into the cave. It will take the team seven gruelling days to get there and back, from the initial 100 metre abseil at the entrance, through to the final survey point at the Great Wall of Vietnam. After the abseil, there's a 350 metre scramble leading to the first of two river crossings. From here, the team need to climb high, following the river through the natural valley of the cave. Then it's a descent down to a level sandy area and to the first of two collapsed areas within the cave, known as dolines. These dolines in the cave beyond is where Beccia hopes to find new life forms and Granger the secret of how the cave was formed. It's then through a narrow section, a rat run, and across the larger doline and onto the second camp. The final part of the cave surveyed so far is the last 400 metres, before the Great Wall of Vietnam. And then it's into the unknown. Limbert's caving team have been exploring Vietnam's caves since 1990. They picked the Cay Bang Massif for a very good reason. It's a huge limestone area with lots of water, ideal ingredients for cave formation. Where there's no limestone, the rivers are trapped within walls of non-soluble rocks holding the water in. When it does find an exit, the results can be large and spectacular. The volume and power of the water help to create these gigantic caves. Add a monsoon climate with heavy downpours feeding the many rivers here, and the chances of water carving out a cave of exceptional proportions are even greater. So far they've discovered 100 kilometres of caves, but none are as vast as this. They suspect some additional factors must be at work here. The team now need to traverse difficult broken terrain to reach their first camp. They're nearly half a kilometre into the cave. 20 metres ahead lies a powerful river. This river runs like a main artery through the first part of the cave. Even at its lowest, 10 cubic metres of water pour by this point every second. It carved this enormous cave, and the team will have to cross it twice. Limbert leads the way. Oh, some evidence of a bit of water comes through here at times. A bit slippy on these. Oh, I can see the river. Whoa! Look at this. This is our river crossing. Oh, then we're underwater. The last bit that looks a bit quick. So if I go up there and belay you... Life and death, really. So, I mean, it has to be right. Worst case scenario, one of these pops, you've got to back up. That's why you've got to make it as safe as possible, really. In full flow, the river could sweep any one of the cavers off their feet. And there's another danger. If it floods, it could rise well above their heads, cutting off their escape route. What's the forecast for the next few days? Well, the forecast is good, but you can never be absolutely confident. Limbert has chosen the timing of the expedition very carefully. Now is the end of the dry season, so the river is low. But any unexpected tropical downpour could see the river rise dramatically in just hours. Let's do it. Right, guys, you can start crossing. The most important thing is you don't allow the river to sweep you away because there's a load of rapids just down there. And we're not really quite sure exactly what's in the river beyond that because we haven't seen it all the way. And there could be all sorts of very sharp rocks and all that kind of thing. And the last thing you want is to get swept down the river because it could be a serious injury or even life-threatening. I'm actually quite nervous about river crossings because I'm quite a slight build. And I've been in flooding case before where when you put your hands on something, your legs just get swept away from underneath you and you can't get them back down to get footing. It was actually really, really dangerous. A major obstacle defeated. Granger can now start to really look around. You can see big formations, big stalagmites over there. Check that one out. This is an amazing cave. We came down those big slopes, we've come through the rocks and we finally made it down to river level. So now we can set up camp here and we can start thinking about what we want to do next in this cave. His head buzzes with questions. Why in a land of giant caves could this one be the largest of them all? What is it that makes a cave so big? There's a lot of stories that we can learn from this cave. To claim the world record, Mountain River Cave needs to be consistently higher, wider and longer than the current title holder, Deer Cave in Sarawak, Malaysia. When you're talking about largest caves in the world, there's no doubt there's some very, very long caves. And if you went on pure volume, many, many caves are bigger than this cave on pure volume. It's pure dimensions of a whole single passage like this that continues for five kilometres is pretty unique in the world. The cave runs in an unusually straight line from north to south without any deviation. So let's get underway, let's get started, okay? Okay. Granger believes the water in this earliest, oldest section of the cave could offer an explanation why this cave is so enormous. Highly erosive, exceptionally acidic water acting on very pure limestone might be the answer. To confirm his theory, Granger needs to carry out a test. Okay, so right now I'm taking a pH measure. I have a pH probe here. We have to leave it in the water for a little while to equilibrate. That'll tell us the pH of this water. pH levels range from 0 to 14. The lower the pH number, the stronger the acid. To be out of the ordinary, Granger is looking for a reading somewhere lower than 7, which is neutral. On the surface, tropical rainstorms deluge the vegetation and seep down through the soil. On the way, the water picks up carbon dioxide, which makes it acidic. High levels of acidity would make the water flowing into Mountain River Cave more effective at boring its way through the rock. Right, so my pH should be finished now, and we have about pH 7.5. So that's pretty typical, maybe a little bit on the low side. The results of the test surprise Granger. The water here is not unusually acidic. This can't be the explanation for the cave's epic size. There's something strange going on here, something which he needs to discover. But finding the right answer to this huge puzzle is his biggest problem. Granger needs to dig deeper. The explanation could lie in how efficient the river is at eroding the limestone. To work this out, he must measure how much dissolved calcium carbonate there is in the water. This will tell him how much rock erodes daily. I'm going to add some acid in here until the liquid turns yellow. And then the way this kit works, I can just read off from the syringe how much calcite is in this water. One more drop. So now it's yellow. OK, we've got it. We only have about 62 milligrams per litre. This means that every litre of water that passes by here carries 62 milligrams of dissolved rock, about the size of a baseball each second. It sounds a staggering amount, but for a cave of this size, it counts for nothing. Like the 7.5 pH, the 62 milligrams per litre result is only surprising because it's so ordinary and unexceptional. It's exactly the result a geologist might expect to find in any limestone cave river. These results don't help Granger understand why this cave is such a monster. His answers must lie deeper within the cave. I've got to rethink my plans a little bit. I've got to think about what this tells me about the cave evolution. We'll keep our eyes out and see what we find. The team need to rest and set up camp. They only have enough food for six days, enough to get to the wall and back. Rehydrating and replacing lost calories has to be carefully managed. Dr Hu Mu Nguyen, a geomorphologist, and Dr Tai Mu Nguyen, a biologist, both from Hanoi University, help Limburg with the organisation. Hopefully the other porters will take all the other bags to our underground camp. This is a great trip. It's an amazing group. Everybody has pulled more than their weight. Beccia goes deeper and further into the cave looking for life. And like Granger, she's getting frustrated in her search. I expected to see a very old fossil cave with lots of dry passages. A cave of this size would normally have many areas where water can't reach, giving various life forms a survival niche. But here, Beccia is discovering that the water is literally washing all traces of life away. Far from her dream of finding undiscovered species, Beccia only sees a barren wilderness. It's a bit of a shame. I had great expectations, but with the wet flooding, it's hard to rethink basically my approach. The team is exhausted and settle down for their first night inside the cave. The next morning, they push on, over a kilometre into the cave. Daryl Granger has explored some of the largest caves in the world. But this one is about to deliver something he's never seen before. Deep in the heart of Mountain River Cave lie the magical and mysterious Dolines. Wow. Here, the cave roof has collapsed inwards, allowing sunlight to flood in, connecting above and below ground with the vital ingredients of life, water and light. This is the world's biggest cave. The rest of this stuff is just building up to it. That is amazing. That's one and a half kilometres away. That is huge. That looks like, you know, it's a hundred metres away, like we're in a normal sized cave. That's a kilometre and a half. It's just, it's hard to even imagine how big that passage is. Here, a unique mini jungle has taken root, 400 metres below the surface of the earth. Seeing the jungle, basically in the middle of the caves, is one of the most beautiful sights. It's just absolutely incredible. It's got to be one of the wonders of the world. It doesn't happen anywhere else on this planet. I think that's just about the holy grail for a biologist. The Doline is an unexpected window for Granger into the structure of the limestone here. It's also a view of Mountain River Cave's Achilles heel. This is a good example of a special type of Doline. This is a collapsed Doline. So that happens when you have a cave passage and then the surface collapses into that cave. And it happened here because we have really thin beds. You can sort of see them on the backside of the wall and they're just not strong enough to hold up this cave roof. At some point in the cave's past, the river ate away at the cave roof where the limestone beds are thinner. The river undermined the ceiling rock so much that eventually the roof collapsed. This collapse is known as cantilever failure. You can think about, theoretically at least, having a piece of rock sticking out like a diving board. You can only stick it out so far before it breaks off under its own weight. And that's exactly what happens. As you make the cave bigger and bigger, you're forcing basically a diving board of rock, a slab of rock to stick out and support its own weight. The limestone beds in this part of Vietnam are especially thick, with very few cracks or weak points. The river tunnels through this landscape of pure limestone. The Dolines brutally reveal the few places where the rock beds are thinner and so weaker. Here, open to the sun and the rain, life has taken hold. Dr. Tai and Annette Boettcher are the first to climb into the undergrowth. She's keen to tap into Tai's expertise in rainforest plant species to figure out how this subterranean wonderland compares with the surface. So thin and tall. Reaching for the light. Yeah, that's what I thought. This is like a lost world. It's basically untouched by humans, which is very exciting. So we're the first people to come here, which is amazing for a biologist. And so it's all mine. I can go around and I can sample things and whatever I find will be interesting and hopefully very different. After drawing a blank in the first part of the cave, the Doline fulfills Boettcher's expectations. Here is an area rich with life, a unique and unexplored ecosystem. This is absolutely fantastic. It's a treasure trove for biologists. And everything that lives here has got here by complete chance. So birds have dropped seeds, plants have sprouted up. And they must manage to flourish in a different world from the rainforest they came from. Here it's darker, more protected from the wind. The floor of the Doline is made up of porous limestone rubble from the collapsed roof. Less light and little water means these plants have had to evolve new strategies to survive. This species outside is evergreen. OK, correct. But here it's fallen down the leaves in the winter, I think so. OK, so you think it's evergreen outside, but it's deciduous in here. So the young leaves are those, the red leaves, are those young leaves? OK, right. Boettcher had hoped to find new species of plants evolved to flourish in this strange underground world. But Tai doesn't see examples of unique evolution. Instead, he thinks the plants in the Doline are showing extreme adaptation. It's a process known to biologists as phenotypic plasticity. In particular, in plants, their outer form is very variable. They can adapt themselves very much to their environment. So there could be two plants that are perfectly the same species, have the same DNA. But because they're in a different environment, they actually look quite different. That's the plasticity bit. So I think we're seeing an example of spectacular phenotypic plasticity. Novel adaptations abound in the Doline. In the tropics, trees are usually evergreen. But down here, they're deciduous, with sparse foliage and narrow trunks. Dr. Tai theorizes that this is how the trees deal with the relative lack of water and shortage of light. These sheer Doline walls enclose this mini jungle and act as a natural barrier to the outside world, trapping pockets of life deep within the cave. Boettcher looks for soil thick enough to lay some surface traps. Alcohol will preserve any creatures that venture in. Now she must try to find any signs of life in the dark cave. But so far, the chances of finding anything of significance don't look good. Could the water that created the cave also be destroying all life within? In the wet, active part of the cave, Granger discovers a clue why the apparently very average water here has carved out a cave so very far from average. He's got a hunch the speed of the water at peak flood could be the key factor. Here, what we see are little scallops that are carved in the wall. They're dissolved in the wall by water as it passes by. He expects to find evidence that the water runs through here at great speed. The faster the water goes by, the smaller the scallops. So if we see big, giant scallops, they can be up to a metre across. That's very, very slow-moving water. Here, they're about an inch across, a couple of centimetres across. From the size of the scallops, Granger can also work out what's known as the Reynolds number, a crucial measurement that tells him how turbulent the water is. Like the acidity test earlier, the findings here are very average and offer no clues as to why this cave grew so big. He works out that the water here travels at only five kilometres per hour, a totally ordinary velocity for any river. So far, he's drawn a blank on his quest, but he might still be able to help Betja on hers. Granger thinks the cave's personality may be about to change for the better. I think what happened is the first doline collapsed, and it's acting like a big dam right in the cave. We pass that doline, and suddenly everything dries up. We don't see the sand. It becomes more normal. It becomes more of a normal, less active cave. And that's where the wildlife comes in, because if it doesn't get flooded out every 10 years or so, it can actually, it has time to evolve. For Granger, the formula for how Mountain River Cave grew so big remains a mystery. This area has the right ingredients for a big cave. Unusually large beds of pure limestone and heavy monsoon rains feeding a deep river. But the water's speed and turbulence didn't reveal anything, and Granger is almost at a loss. The surveying team looks for answers too. That's probably a 500 metre echo. So if we ever push in a cave, and you get to end of a cave or end the exploration for a day, we often give it a big oi, just to see if it's still continuing. And we know we can turn around and come back to another successful push the next day. For this expedition, they need to use a more sophisticated way of measuring the cave's size. Cave surveying used to be a primitive process where everything was done by hand. Now, digital laser readings cut out the tedious legwork and provide precise measurements. They must survey every part of the cave, and every measurement must be exact to the nearest centimetre. When it comes to claiming the title World's Biggest Cave, it's length, height and breadth that counts for Limbert and his team. The laser sends a beam across to a designated point called a survey station, which sims marks with a small light disc. They measure distance, height and angle. It's painstaking work. 29.9, minus 3, 099. Left side. The laser beam picks out the highest point in this section of the cave. It measures 100 metres. The Statue of Liberty could fit in here quite comfortably, with 7 metres to spare. The widest section surveyed so far is 103 metres. This passage continues for another kilometre. Room for a small plane to fly through the cave, or even a whole display team. Very good. So if we go back to the original station, and then we get some legs straight down there. OK. Can you shine your target? Limbert knows this cave is gigantic, longer and wider than Deer Cave, but he still needs to find an area that's higher to be sure of the title. Just up a bit, Jonathan. Light on. They're now over four kilometres in, and Deer Cave still holds the title of the world's biggest cave. Hope lies with the wall, but they have just three days left, and there's a problem mounting. The dolines bring in light, but they also allow the weather from above to seep down into the cave. Jonathan. Can you come a bit closer? Please. We've got a problem with the clouds now. It's extremely accurate if there's no cloud in the way, which there is in this case, across the far side. So this is one thing that does make it difficult, one of the peculiar hazards of surveying in a cave this size. The poor visibility means the team must stop the survey. It's an awful setback. They're running out of time to get to the end of the cave. Limbert makes a tough decision to save time. He sends his two best climbers, Sweeney and Clark, ahead to set the rigging on the treacherous wall, so the rest of the expedition can follow tomorrow. Ahead and still to be conquered, the Great Wall of Vietnam. If they can't get over it, they may never be able to claim the world record. In 2009, the explorers estimated the wall to be around 14 metres tall. Now Clark and Sweeney think it's much bigger, more like 30 metres, about the height of a nine-storey building. They have the gear for the challenge, but the wall is going to be a longer climb than they expected and much more dangerous too. Geologists call it a flowstone, rock made weak and fragile by water dripping from the ceiling. The first 15 foot is this, which looks very good, but if you... That's what it is. This wall could prove the cavers and the mission's downfall. That's just mud and gravel, is that? It soon becomes clear this cave is not going to give up its secrets without a fight. Just crumbling away. What's that? It's crumbling away. Sweeney and Clark have been building up to this for over a year. It's something that they're particularly interested in, is climbing and climbing the wall. What we're climbing in is a very soft mud calcite, and so normal bolts would just fall out. It's going to be a long and difficult climb. Back at the camp, the extreme conditions are taking their toll on even those who are not battling the wall. What can be quite difficult is just living in what can be quite squalid conditions. You've got to keep things clean. It's very dirty at times. You're getting sand and dirt in everything, be it your caving equipment, all those clothes. Feet are a big problem because you're continuously wet or sweating or going through rivers. We often get what we believe is a fungal infection of the feet, so we tend to put antifungal powder every evening, keep us feet nice and dry to stop this happening, and touch wood. We're OK at the moment. As the expedition party settle down to rest, there's another unexpected problem. A miscommunication with the surface. Limbert needs more porters, but they're running out of food. We're just trying to sort out how we can organise a porter situation and pots and pans and food and all that. OK, I think it would be ideal to have porters down here and camp down here, as long as they've got some blankets and they've got some food, because we've not got enough food to spare for porters, I'm afraid, over. Well, the porters don't have any food themselves. They said that the food was supposed to come from you. Yeah, we never really envisaged having underground porters camping, so we didn't cater for that. Are there no rice or any noodles at all on the surface that they can bring down, over? They've got no food. We've got a few problems at the moment. We've only got two porters down here. We're going to require more. We've got a lot of loads. It's going to be a little bit tight on the food. We're looking like we've got 24 people, but we've only got 18, 20 lots of food. Well, the thing is, we've got six of those big packets of pasta for each of two meals, and that'll stretch. We've had a good noodle breakfast. That'll do us for the next day. Excellent news. Yeah, that's all got to go out. That's got to feed them for four days. Betcha's search for life continues. She came to Mountain River Cave to find new species, but with only two kilometres to go before the wall, she's found nothing extraordinary. She has one more chance. A promising new sample area, a dry passageway. That's where the opportunities are with this fantastically big cave. So I can go into those and look for those tiny critters that will be evolving in the large, dry side passages in the alcoves. Regular flooding in large parts of the cave mean that it's really difficult for new life to gain a foothold. But in these dry alcoves, Betcha thinks she may find isolated species which have had time to evolve, to flourish in complete darkness. The smaller and more primitive the species, the better equipped it is to rapidly evolve. And it's not just because they're simpler genetically. Evolution takes a long time. If you have something like two or three generations a year, you can evolve at much greater speed. And mammals don't tend to have those generation times. So, you know, a white blind bat in the cave is unlikely. Because of the cave's sheer size, species can easily get cut off from each other. It's then possible for weird and wonderful offshoots to evolve. Oh, my word. Look at these. This is the first white woodlouse that I've seen ever in the cave. These are basically the same blueprint woodlice as we know from outside, from the surface. But on the surface, they would be dark coloured, probably dark grey, slate grey or black. This is not something I expected to find here, so I'm very pleased. Come on. After finding nothing at all in the great expanses of the cave, Betje has struck gold, finding a creature common on the surface that appears to have evolved to survive in this unique twilight world. I'm almost 100% certain that this is a new species. And the reason being is to have a new species, you need isolation, geographical isolation. Well, this is a pretty isolated place. They look different. There's nothing nearby that I've seen that's got no pigmentation. It's a woodlouse, all right, but it'll be a new species of woodlouse and it'll be specific to Hang Song Dong. It's about as good as it gets. It's a fantastic discovery for Betje. A species that has adapted to its environment and survived the great floods of water that helped create this cave. Well, there's absolutely no chance that anybody's ever seen these creatures before and that's what really excites me. It's a wonderful discovery. The woodlouse is a clear signpost to Betje to what's happening here. In the cave, the organisms that hit on the best adaptation to the prevailing conditions have a better chance of survival. But the key is being able to keep up with this ever-changing environment. When I set off, I had actually fairly low expectations. So finding the white woodlice is much better than expected. I'm very excited about it. The woodlouse evolutionary path has enabled them to survive and prosper in an environment never seen anywhere else. I would expect a new cave species probably to be pigmentless, so looking white to us and be blind. I don't think that the woodlice we found are blind. They don't seem to have rudimentary eyes and they can certainly sense light. So they are adapted to the cave environment. They just haven't gone the full way yet. So there's room for them to evolve more and the next step would probably be for them to lose their eyesight. Betcha has found a new species. She's now joined a select group of scientists that can lay claim to making a once-in-a-lifetime discovery. The finding and documenting of new life. But none of this would be possible were it not for the truly awesome scale of this rocky, lost world. Back at the camp, time is running out and the last of the food is being served. While Sweeney and Clark continue in their battle to secure rigging on the wall, ready for the whole team to follow. If they don't succeed, the team can't complete a definitive survey. Limbert and the rest of the party wait nervously for news. As time runs short, Granger is convinced he's near to cracking the geological puzzle of why this cave is so large. Huge volumes of water, pure limestone and a tropical climate are important factors. But this is not enough to explain why this cave is so big. In a bid for answers, he returns to his maps of the region. So this is a digital elevation model which shows the area that we're in. Right in here, this is the Kebang National Park. It turns out the critical evidence comes not from geology, but from old-fashioned geography. Granger spots an old fault line on the map. It seems to mirror the underground route plotted by Mountain River Cave. This could be the answer he's looking for. You know, the real light bulb moment is when I saw the fault line and then everything kind of clicked. What I noticed are these straight lines in here, they go due north and south. And remember the cave here goes north and south. And I realized we're right on that fault right there. That's exactly what formed this cave. There's that same fault right up there. Once a river ran where Mountain River Cave runs today. There was a deep vertical crack in the earth's crust, a fault. Eventually, the river water eroded the limestone and the raging river went deep underground. Once it filled the crack and found the exit, the river then started to erode the rock following the path laid out by the fault and a gigantic cave was born. But there's another key factor. The fault was narrow. When the water began to carve this mega cave millions of years ago, it was the shape of the fault that gave it strength. The reason the caves fall apart, the reason they break down is often because they get too wide and the limestone roof can't support that long a length. Here, the cave remains relatively narrow within the straight jacket of the geological fault, reducing the possible risk of collapse and allowing the cave to continue to grow in a continuous straight line. The fault is very vertical and that has also influenced the cave development and helped turn it into what's probably the world's biggest cave. And that's because the walls are perfectly vertical. They're nearly vertical here. And so the cave is very, very tall. The 400-meter-high dolines show what happens when a cave weakens. Grainger has every piece in place. Unusually thick, pure limestone. A huge catchment area feeding a large river. An underground fault line, the right shape to contain a stable cave passage. And finally, a river on the limestone surface that made a cave at the first opportunity. A perfect formula to build a gigantic cave. The title of world's biggest cave could rest on what the team find beyond the wall. But first they need to know if Sweeney and Clark have managed to climb it. Limbert finally gets the news he's been waiting for. The advanced team has reached the top. The height, or pitch, of the wall is even more than anyone anticipated. Absolutely fantastic. The pitch is, well, we reckon it's something like 80 meters. This is used about 80 meters of rope. 80-meter pitch? Great news, yeah. That's the main objective of this trip is to get up that wall and we've done it. This morning I was very doubtful we were going to do it. I thought we were going to be too tough. But they managed to get up. They've done a brilliant job. 80 meters is a big climb in those conditions. The good news brings speculation as to what lies beyond the wall. I'm not going to be surprised if that's the end of the cave. I know Howard thinks it's going to turn left. My inclination is it's going to go straight there and terminate. But, you know, that's what we're going to find out. The final hurdle now awaits Limbert and his team. As they push on to join Sweeney and Clark at the Great Wall of Vietnam. It took almost two days for those guys to bolt up there. It's very fragile rock, so you need to be very, very careful. They've done a great job. My greatest feeling was one of relief, which perhaps indicates how serious the climb it has been. You're standing up there and you're looking down and all this and it's absolutely tremendous. They finally arrive at the wall. It towers above them. This is what they've been working for. Their ultimate destination. And now they're just a climb away from a possible world record. Limbert and Sims are looking for a section of the cave that beats the highest point in Deer Cave. They haven't found it yet. And if the cave ends here, the world record may never be theirs. It will be very, very dangerous. So we'll have to be very, very cautious going up the wall. It's only for experienced cavers, really. Limbert takes the difficult decision that only the surveying team will go up the wall. It's too dangerous for Granger and the film crew to follow. This is it. We're about to go up and push into the unknown, as they say. Who's going up first, then? Sweeney. I'm ready to rock. You just have to hold a bit of tension for the first bit of fire. We're all on tenterhooks waiting to get over this wall to see what's next. The film crew rig one of the cavers' helmets with a camera to record the climb. They can only watch as Limbert leads his team up on the final stage of the journey. It's a whole team effort. I mean, it isn't just a few people going up a wall. It's the whole build-up. It's the sort of year of planning. It's all the prep. It's the porters. It's absolutely everything that gives a few people the chance to do something like this right at the end. The cavers manage to climb the 80 metres up the Great Wall. Absolutely fantastic. This is something else, is this. Look at that. They have succeeded in the most difficult part of the expedition. The Great Wall of Vietnam, this huge flowstone, the height of a 25-storey building, lies conquered at last. But the elation is quickly followed by disappointment. As Granger had suspected, Mountain River Cave does indeed end here. An exit to the outside jungle lies just 300 metres beyond the wall. With no more cave to survey, this final measurement of the passage from the top of the wall to the ceiling has to beat Deer Cave's highest point of 175 metres. They've just climbed 80 metres and now need a reading of over 95 to claim the title of the world's biggest cave. 45.8. Sims reads the final, crucial measurements. 188. Can you just do the height of the passage there, Jonathan? Yeah, absolutely. Massive. Absolutely huge. Finally, they get the readings they've been waiting for. It's 96.4. 96.4! That's incredible. A spot there, will you? Jesus. That's incredible. Mountain River Cave is the world's biggest cave. With a passage continuously higher, wider and substantially longer than any other cave in the world. Staggering. Absolutely staggering. And, well, I think there's not very much doubt now that this is the world's biggest cave. It's a pretty tremendous achievement. It's a fabulous cave. Really, really fabulous cave. Really enjoyed ourselves. We've had a great time. We're having a really, really good time. It's just mind-blowing stuff, is it? It really is. And it just doesn't get much better. I mean, the expedition has been a great success. It's achieved everything it wanted to do. And you can't ask for any more than that. This has been a tremendous experience for me. I feel very lucky to have been invited on this trip to see the world's biggest cave and try to figure out the science behind this cave. Just very privileged and lucky to be along on this trip with Howard and his team.