00:00A world hidden beneath our feet for 3 million years.
00:04Sun Dung.
00:05The largest cave on earth.
00:08When I say largest, I mean it in a way that breaks your brain.
00:12You could drop a 40-story skyscraper inside this cave and it wouldn't touch the ceiling.
00:18A Boeing 747 could fly through its widest chamber with room to spare.
00:23This single cave could hold 15 great pyramids of Giza inside it.
00:28That's not an exaggeration, that's the actual measurement.
00:32And it was discovered by accident.
00:35In 1990, a Vietnamese logger named Ho Con was working in the Phong Nha Co Bang National Park when a
00:41heavy storm hit.
00:42He ducked into what he thought was a small shelter.
00:46But he felt something strange, cold air rushing up from beneath his feet.
00:50He pushed aside some vines and found a dark hole in the ground, with wind howling out of it.
00:56He could hear water flowing far below.
00:59He went in briefly, saw the entrance chamber, then left.
01:03When he tried to find it again, he couldn't.
01:07The jungle swallowed it.
01:09For nearly 20 years, he searched.
01:12British caving expeditions heard about his story and begged him to help them find it.
01:17He went into the forest again and again.
01:20Finally, in 2008, he stumbled back onto that entrance.
01:25In 2009, he led the first expedition team inside.
01:30What they found changed everything we knew about caves.
01:33The entrance alone is a vertical drop of about 80 meters, roughly 30 stories down.
01:39You don't walk into Sun Dung.
01:42You rappel into it.
01:44And when your feet finally touch the floor, you look up and realize the ceiling is so high above you
01:49that clouds are forming up there.
01:51That's right.
01:53Clouds.
01:55Inside a cave.
01:56Here's how that works.
01:59The cave is so massive that it creates its own microclimate.
02:03Cold air settles at the bottom.
02:05Warm, humid air enters through collapsed sections of the roof.
02:10When they mix, water vapor condenses into clouds.
02:14Real clouds, hundreds of meters underground.
02:18Sometimes it rains inside the cave.
02:21An underground rainstorm.
02:22In a cave.
02:25Those collapsed sections are called skylights, two massive openings where the ceiling fell in about 50,000 years ago.
02:32Light pours through these holes like theatrical spotlights, and beneath each one, a full jungle has grown.
02:39Not moss.
02:41Not ferns.
02:43A real rainforest with trees reaching 40 meters high, competing for sunlight just like they would on the surface.
02:50Beneath those trees, the cave floor is littered with something odd, strange rock formations that seem to reach toward the
02:56light.
02:57And here's where it gets really interesting.
03:00These rocks aren't growing toward the light.
03:03A thin layer of cyanobacteria and bacteria lives on their surface.
03:09These microorganisms need photosynthesis.
03:12Over hundreds of years, as they grow and multiply, they slowly dissolve the stone beneath them, etching tiny spikes into
03:19the rock.
03:20Every single spike points in exactly the same direction, toward the sun.
03:25It looks like the rocks themselves are moving, reaching for the light.
03:29Now walk deeper.
03:31Past the skylights, into the permanent dark.
03:35The underground river that carved this cave is still flowing, still acidic, still eating away at the rock.
03:42In the pools along its banks, biologists have found life that doesn't exist anywhere else on earth.
03:48Fish with no eyes.
03:50Not blind, they never grew eyes at all.
03:54Transparent bodies, completely colorless.
03:58Centipedes, millipedes, spiders, seven new species so far, all with the same features.
04:05No eyes.
04:06Pale white.
04:08Translucent.
04:10Because in a world that has seen no light for millions of years, evolution simply removes what you don't need.
04:16Eyes are expensive.
04:18So nature took them back.
04:20The creatures here are a living demonstration of natural selection.
04:24Near the light, organisms grow toward it, carve toward it.
04:29In the dark, organisms let go of sight entirely.
04:33Same cave.
04:35Different neighborhoods.
04:37Different rules of survival.
04:40In 2010, the expedition team hit what they thought was the end.
04:44A 60-meter wall of solid rock, almost vertical, blocking the tunnel completely.
04:50They called it the Great Wall of Vietnam.
04:53They had no climbing gear, so they turned back.
04:57The next year, they came prepared.
04:59They climbed it.
05:01And on the other side, the cave just kept going.
05:05It had fooled them.
05:06In 2019, divers followed the underground river deeper and found a connection to another massive cave system called Thung.
05:14If they're fully connected, Sun Dung gets even bigger.
05:18An explorer once said this is like discovering a new mountain peak on top of Mount Everest.
05:24We still haven't mapped the whole thing.
05:27We still don't know where it ends.
05:29Here's the sobering truth.
05:32The river that carved this masterpiece is still cutting.
05:35The ceiling has already fallen in two places.
05:39One day, geologically speaking, soon, the entire roof will collapse.
05:45The largest cave on earth will become a very large valley.
05:49Three million years of hidden majesty, slowly returning to rubble.
05:53But right now, it's still here.
05:56A complete world, with its own weather, its own forests, its own rules of evolution.
06:03Creatures that abandoned light.
06:05Rocks that reach toward it.
06:07Clouds that form underground.
06:10While we look at the moon and send rovers to Mars, this entire hidden world lay beneath a jungle,
06:15waiting for one man to step on the wrong patch of ground, feel cold air on his face, and look
06:21down.
06:22That's Sun Dung.
06:23The largest cave on earth.
06:26And we still haven't found its end.
06:54And we still haven't found its end.
06:57The largest cave on earth.
07:00The largest cave on earth.
07:02The largest cave on earth.
07:02The largest cave on earth.
07:03The largest cave on earth.
07:03The largest cave on earth.
07:03The largest cave on earth.
