The World's Largest Cave: Inside There Are Forests, Rivers and Clouds

Sandan, the largest cave on our planet. This cave is so massive that it has a completely self-sustained ecosystem with its own climate conditions. Hundreds of meters underground, the clouds you're used to seeing in the sky form inside this cave. Sandan even has an untouched prehistoric rainforest hidden far below the surface and a vast underground river. From transparent fish to colorful monkeys, over 250 endemic plant and animal species live in this underground world. Seven new animal species were recently discovered here and are unique to this place. Stretching 9 kilometers in length, this cave holds 38.5 million cubic meters of space. And its passages are so enormous that a 40-story skyscraper could fit inside. And its giant stalagmites are rising like towers of ancient cities. Walking inside, it's impossible not to feel as small as a tiny bug. Can a rock move on its own? Sandan even contains phytocarst rocks, which move towards the sun like semi-living organisms. Rare cave pearls and many other unique formations are also found here. For 3 million years since its formation, the world had no idea about this cave's existence until it was fully explored in 2009 in Vietnam. Now, it's time for you to see the largest cave on Earth. Individual access is forbidden to protect both visitors and the fragile environment. This expedition is operated by Oxalis, the only authorized agency. To reach Sandan, we've trekked through the jungle for 9 hours. And now, we must pass through N Cave, the world's third largest cave. This cave serves as a gateway to the world's largest cave. It's crucial to be fully prepared for every possibility. So, before setting out on this journey, we did deep online research about Sandan and its surrounding area to learn about everything that we would need to know. Now, we're heading towards the beach inside the cave. Over the centuries, water from nearby rivers gradually carved this place. The ceiling is 145 meters high, and the cave is 200 meters wide. During the flooding season, much of this empty space is submerged underwater. You can understand from the basket over there. The water levels rise so much that it can carry objects up to that level. With a breathtaking lake in front of us, it was impossible not to take a quick dip. At dusk, the sound of swift birds echoes in the cave. Because of their short and weak legs, these birds can't take off from the ground and must jump from a high place to fly. If they are trapped on the ground, it means death is certain for these birds. This one was unlucky and waited to die. At sunrise, we're ready for the journey to Sandan. If we call this cave a neighborhood, then Sandan is a massive city in comparison. We will move to the darkness now. I can't help but wonder what awaits us in Sandan. As we walk, dozens of crickets on the ground start jumping from side to side. The ground is covered with centuries of bat guano. Above us, countless bats hang in deep sleep. But some are awake and constantly defecating on us, so it's not a good idea to look up. That's how my arm looks. We pass by formations that look like travertine. The limestone steps take us to the path that leads to Sandan. Wow. The view here is amazing. You can see some water coming down from both sides. En Cave welcomed us for a night deep in the jungle. Now, it's time to meet Sandan Cave. It's trying to come to me, you know? It's moving towards me. It knows what to leech on. It got too big. Beneath the soil we walk on lies a 450 million year old limestone formation, which began forming long before the existence of humans or even dinosaurs. This calcium-rich limestone is composed of shells and skeletons of ancient sea animals that sank to the ocean floor and gradually compressed over millions of years. However, since calcium dissolves easily in acid, acid rain slowly erodes limestone and forms caves over time. This is how the world's largest cave was formed by rain-dissolving limestone. A challenging final climb brings us down to the grand entrance of Sandan. As we move deeper, everyone's switching on their headlamps now. This path will lead us to the Hope and Vision Passage. In this 9km long cave, there is so much to explore. We might be the first to record newly discovered animals. This is the entrance of Sandan Cave. You can see those people look tiny at the entrance. Even just a few steps in, we're surrounded by incredible cave formations. The water, rich in minerals, flows down from the ceilings or walls and builds these beautiful rimstone pools over time. The very first animal greeting us is a cricket. We are making our way through the rocks. Some stalagmites are sparkling in pure white because of the mineral type they're composed of. It looks like a diamond. Yes, it does. There's a huge stalactite in bronze color resembling frozen waterfalls. The orange-brown hues suggest the presence of iron or other minerals mixed within the limestone. Some stalactites have actually broken off from the ceiling and fell down to the ground. That one in the middle is a fault line. It was formed because of tectonic movements of the earth. Sandan's enormous size is not just the result of water erosion, but also the fractures caused by tectonic movements, which created pathways for water to flow deeper. There's a huge river in this part of the cave. If you fall, you can't swim. It's so fast. Heavy rains feed many rivers here. And a large and fast river now flows through the cave. How did this come to life? We are like hundreds of meters inside the cave. There's no sunlight. Heavy floods can bring soil and plant stems into the cave. That's why we see a sprouted seed here. Unfortunately, without sunlight, they won't survive for long. The flood also brought spores in, so now there are mushrooms growing here. The moisture and darkness are perfect for them. Just yesterday, the water reached up to here. It carried a lot of mud with it. It's so recent, as if the flood just happened here. Signs of the incident are still visible. We reached the Hope and Vision Passage. This is the largest part of the cave. I'm standing next to the tallest stalagmite in the world. It is 80 meters high. It took an average of 800,000 years for these minerals to accumulate on the ground as water dripped from the ceiling and formed this massive stalagmite. The reason it got so huge is because it grew in a part of the cave where the ceiling is over 170 meters high and more than 130 meters wide. So, this part of the cave is as large as a 40-story skyscraper could fit. And even a double-decker aircraft could fly through. You can see the light coming from the first building very far ahead. That's where we are heading. We spot cloudy-like white patches on the ground. What is this? A spider web. When they sense vibrations of a trapped insect, the spider jumps out, bites the prey, and returns to the funnel to eat. These are sand towers. When water drips from above, the parts under the pebbles are protected while their surroundings erode. And in the end, it forms these towers. It looks like a small metropolis. If you carefully look at it, there's a guy standing on the top of that rock. Our teammate in the far distance is now standing on a stalagmite of 60 meters in length. We move 3 kilometers forward in the cave. This is our camping site. Imagine sleeping here, looking at this view. From here, you can partially see the first dew line. It might look like the cave's exit, but it's actually the first of two massive dew lines. And from here on, things will get more incredible. The temperature difference between the outside and the inside of the cave forms mist and clouds inside the cave. The area beyond them is called Watch Out for Dinosaurs. We will explore that place tomorrow. For now, it's time to rest. Now we are going to first dwelling. This is the next day. There will be a lot of things that we will see today. Let's go. We are crawling through narrow passages to get to the first dew line. A very narrow point. These sticky threads are traps belonging to cave worms. They produce a blue-green light from their tail to attract flying insects to these sticky threads. Once an insect gets stuck, the larva pulls up the thread with its mouth and eats its prey. This limestone is the oldest limestone in Southeast Asia. It is more than 400 million years old. While some sea creatures become part of the limestone, others mineralized and ended up as fossils like you see in this example. We'll get into the details of this fossil later. We're almost there. Sun! Well, it takes for water droplets to reach the ground for like 20 seconds from up there. What is the height of this wall? It's around 140 meters. Everything is huge here. When you look up, you can see the trees of the rainforest surrounding the opening. But right now, we are meters below and still inside the cave. The reason why this hole exists is that hundreds of thousands of years ago, the river flowing inside Songdong eroded the limestone and weakened the roof. The roof couldn't hold up and collapsed, creating this giant hole above. So the light gave life to these algaes and mosses. In the afternoon, we'll go to Dolin No. 2. It's very different. You'll see a real jungle there. But first, let's see the first of the last two incredible formations here. This is also a stalagmite. But it's a different one. It owes its unique layered shape to the waterfalls. These are phytocarst formations. They are rare limestone formations shaped by the growth of algae, mosses, and microorganisms together with calcium carbonate. Since both biological and geological processes shaped them, they're like partly a living and partly non-living. Here, you can see the phytocarst formations moving toward the sun like a hand extending from the cave. Climbing one kilometer down and up in the dark, we finally reached our next destination. This is the second Dolin, much bigger than the first one. This looks amazing. All of a sudden, we are in a forest in a cave. Look here. They're so dense. The roof of the cave at this point collapsed as a result of the intersection of two main faults, forming a dough line on the surface 252 meters above where we are. The collapsed cave roof allowed sunlight to enter, giving rise to an entirely new ecosystem. Despite extreme conditions, plants and animals have managed to adapt and turn this place into a unique underground rainforest. As we move forward towards the light, the height of the plants grow. The soil makes it perfect to form a rainforest. A large portion of the soil here consists of bat guano. It has accumulated for millions of years and is highly fertile. The soil here is deep and full of small holes. There is an insect in each hole, and the insects are making traps in these holes to catch animals. This jungle is huge. The biologists have found more than 200 species of plants and almost 60 species of animals living in this area. There are some monkeys on trees. This might be the only cave where monkeys live. Most primates typically prefer trees as their habitat, and cave environments are not suitable for them. However, this extraordinary cave has become a new living space for certain species. We came across a centipede. And snakes are living in this forest as well. The sound of this forest is calming. Even the explorers, who first came here in 2009, thought they'd found the exit for a moment. Remember that we are in a cave. This cave has its own rainforest. All of these trees get thinner but taller to reach the sunlight coming from above. That's why all of them are very thin, but very tall compared to their width. Certain trees reach heights of 40 to 50 meters. It's because the trees prioritize height to maximize sunlight absorption. This is the remains of a snail. Monkeys catch these snails and break them with stones, then eat them. As we move through the cave, we start coming across signs red-shinned dukes have left behind. Look, so many snails. Most of them are broken and probably eaten by a monkey. We kind of forget we're in the cave because it's so massive. This is where we are going to camp tonight. It's our fourth and final day in the cave. Today, we navigate through a water-filled passage before reaching the Great Wall of Vietnam. The stalagmites here look fascinating, almost like the work of a sculptor. A pool of water surrounds this column and fish are swimming in it. These fish have no eyes as they have adapted to live in this total darkness. Instead, their other senses got stronger over time. See how it doesn't give any reaction to the shadow of my hand or the movement? It doesn't see anything. Like the fish, this cricket is completely blind. It doesn't see me. These super rare formations are called cave pearls. They are usually one centimeter in diameter, but here, since they are too old, some of them are as big as a tennis ball. Just like real pearls, they form around a tiny nucleus, such as a grain of sand, with layers of minerals slowly building up over time. They need a few hundred years to be fully formed. As we push further into the cave, something unexpected catches our eyes. These are deer bones. They're estimated to be 500 years old. There is one last challenge ahead, an 80-meter-high karst wall called the Great Wall of Vietnam. To reach there, we must also pass through a 600-meter-long Passchendaele Passage. The passage is now filled with water and is turned into a breathtaking jade green lake. It's so high that the light does not reach the very top. After the wall, the light from the exit is visible from 500 meters away. We have explored the world's largest cave from one end to the other, yet on the fourth day of the exploration, one mystery still remains. Which ancient creature, the fossil we saw in Son Doong, belongs to? I'm opening Opera Browser and quickly navigating to the cave's website to check the related article. Here we'll find the same fossil's image and its description. I'm gonna press Command Slash or Control Slash to open ARIA, it's a built-in AI tool. Let's ask it to highlight the key points in this article. The article mentions two different coral fossils, but doesn't specify which image belongs to which creature. Luckily, ARIA has image recognition feature. I'm gonna upload the image and ask again. It identifies the fossil we saw as Tetrachoralea. Let's check for more information on this sea creature. First, I'm using the Tab Islands feature to effortlessly group tabs by topic and save space. I can expand or collapse them as needed. I'm splitting the browser window to view both websites side by side. It appears that Tetrachoralea or Rugosa went extinct 300 million years ago. This carnivorous species is believed to have had tentacles to catch its prey and grew up to 10 cm. Because it is extinct today, we don't know exactly what this animal looked like, but with ARIA's image generation feature, we can create a possible visual for this animal. Here's an illustration of a living Tetrachoralea drawn as a colony with tentacles. By the way, Tab Traces feature, as you can see here, help us see our recent visited tabs by darkening the underscore. The darker the underline, the more recently the tab was visited. Using Opera's smart tools, I believe that we identified which creature this fossil belongs to. Opera also enhances its workflow with a floating music player and customizable themes with many other features. Download Opera now from the link in the description box or pinned comment to take your browsing experience to the next level. Thank you for watching. See you in the next one. Ruhi Cennet was here.