[00:40.04]This is our planet's final frontier, [00:44.24] [00:46.40]an inner world where only the most adventurous dare to go. [00:52.20] [01:43.04]Beneath our feet [01:44.44] [01:44.56]are countless miles of cave shafts and passages. [01:49.48] [02:05.20]The Cave of Swallows in Mexico, [02:07.60] [02:07.72]400 metres to the bottom, [02:09.76] [02:09.88]deep enough to engulf the Empire State Building. [02:13.76] [02:20.64]This is the biggest cave shaft in the world. [02:24.00] [02:24.08]Yet these depths were first explored [02:26.76] [02:26.84]only two years before man landed on the moon. [02:30.48] [02:33.76]Today caves remain the least explored places on Earth. [02:39.00] [02:39.20]However, human beings are seldom the first [02:42.56] [02:42.68]to reach these black, damp places. [02:46.20] [02:50.48]Here, live some of the strangest and least-known animals on the planet. [02:56.88] [03:28.84]This galaxy of little lights is created by thousands of living creatures. [03:35.52] [03:39.76]Any animal that lives in a cave has to cope with complete blackness, [03:44.68] [03:44.76]but in New Zealand some have turned this darkness to their advantage. [03:50.08] [04:02.40]A silicon strand is lowered from the ceiling, [04:06.20] [04:06.32]alongside hundreds of others. [04:09.00] [04:16.24]Beautiful though these threads are, [04:18.44] [04:18.60]they have a sinister purpose. [04:21.56] [04:23.00]This is a cave glow worm. [04:26.36] [04:26.60]To trap its prey it goes fishing with a line of silk. [04:31.40] [04:38.00]The silk comes from glands in the glow worm's mouth [04:42.08] [04:42.20]and is loaded with droplets of mucus. [04:45.36] [04:59.20]Each glow worm produces dozens of these threads. [05:03.68] [05:06.12]Once its lines are set, [05:07.96] [05:08.08]the glow worm hangs from a mucus hammock [05:11.08] [05:11.20]and waits, like a patient angler. [05:14.64] [05:18.88]But the glow worm doesn't leave everything to chance. [05:21.92] [05:22.12]That ghostly blue light is the result of a chemical reaction [05:26.12] [05:26.24]taking place inside a special capsule in its tail. [05:30.60] [05:31.40]The light literally shines out of its backside. [05:34.44] [05:34.56]It's a lure for attracting prey. [05:37.76] [06:04.44]Insects seem irresistibly drawn towards the source [06:08.76] [06:08.88]and then get trapped by the sticky lines. [06:12.64] [06:21.52]Once stuck, [06:22.92] [06:23.04]there is no escape. [06:24.76] [06:30.04]Now it's just a matter of reeling in the line [06:33.92] [06:34.20]and slowly consuming the catch - alive. [06:38.00] [06:53.20]By ensnaring the insects that hatch in this cave, [06:56.16] [06:56.28]these glow worms have solved the biggest challenge [06:59.24] [06:59.36]that permanent cave dwellers face - [07:01.56] [07:01.68]finding a regular and reliable source of food. [07:05.12] [07:12.28]One kind of rock makes this whole underground world possible - [07:16.80] [07:16.92]limestone. [07:18.36] [07:18.72]Most of the world's caves are found within it [07:21.52] [07:21.64]and it covers nearly 10 percent of the earth's surface. [07:25.20] [07:31.20]Limestone is composed of minerals derived from marine shells and corals, [07:36.72] [07:36.80]so although this rocky escarpment in the United States [07:40.12] [07:40.20]is now hundreds of metres above sea level [07:43.04] [07:43.16]it was actually formed under water. [07:46.12] [07:53.12]The limestone towers of Vietnam's Ha Long Bay [07:56.68] [07:56.80]are a reminder of this link with the sea. [07:59.64] [07:59.76]Originally, this whole area would have been one solid block of limestone, [08:04.16] [08:04.24]the base of a coral reef. [08:06.48] [08:16.84]In Borneo, rain has sculptured the limestone into extremely sharp-sided pinnacles. [08:23.88] [08:36.84]But the dissolving power of rainwater [08:40.00] [08:40.08]has other, much more dramatic effects underground. [08:44.40] [08:55.00]Rivers that flow over limestone often seem to completely disappear. [09:00.08] [09:13.92]When the water reaches the more resistant bed of limestone [09:17.20] [09:17.32]its course is altered. [09:19.52] [09:23.72]Once underground, [09:25.12] [09:25.24]the water takes on a new, more erosive power. [09:29.28] [09:34.80]During its journey from the surface [09:37.04] [09:37.16]the water absorbed carbon dioxide from the soil [09:40.36] [09:40.44]making it mildly acidic. [09:42.48] [09:42.72]And over millions of years this acid eats away the limestone [09:46.92] [09:47.04]creating a maze of caverns and passages [09:50.44] [09:50.56]that sometimes go on for miles. [09:53.12] [10:31.72]This is the biggest underground river passage in the world, [10:36.80] [10:36.88]so big a jumbo jet could fly through it. [10:40.92] [10:41.04]It's Deer Cave, in Borneo. [10:44.00] [10:56.00]The sheer size of Deer Cave [10:58.04] [10:58.12]allows some animals to gather there in huge numbers. [11:02.24] [11:09.48]A staggering 3 million wrinkle-lipped bats live here. [11:14.56] [11:18.00]The bats roost high on the walls and ceilings [11:20.80] [11:20.88]where they're well protected from the outside elements [11:23.28] [11:23.36]and safe from predators. [11:25.36] [11:33.36]And while they're up here [11:35.00] [11:35.12]the bats produce something very important. [11:38.56] [11:43.00]This hundred metre high mound is made entirely of bat droppings - [11:48.28] [11:48.40]guano. [11:49.48] [12:11.40]Its surface is covered by a thick carpet of cockroaches, [12:16.92] [12:17.20]hundreds of thousands of them. [12:19.48] [12:24.80]Caves are one of the few habitats on Earth not directly powered by sunlight. [12:30.80] [12:30.92]In the absence of plants this food chain is based on a continuous supply of bat droppings. [12:39.20] [12:50.96]The cockroaches feed on the guano [12:53.68] [12:54.16]and anything that falls into it. [12:56.72] [13:23.20]The droppings also support other types of cockroaches [13:26.84] [13:26.96]which spend part of their day resting on cave walls. [13:30.68] [13:34.80]These in turn become food for giant cave centipedes, [13:39.36] [13:39.44]some more than 20 centimetres long. [13:42.28] [13:45.88]Bizarrely, there are crabs here, too, [13:48.36] [13:48.44]sifting through the droppings for nutrients. [13:51.44] [13:58.40]All these animals spend their entire lives within the cave. [14:02.44] [14:02.52]They're totally dependant on the digested remains of food [14:06.40] [14:06.48]that's brought here from outside. [14:08.68] [14:35.00]Each evening in just two hours [14:37.56] [14:37.68]three million bats leave the safety of the cave [14:41.20] [14:41.32]to hunt for insects in the forest outside. [14:44.56] [14:47.08]But not all will return. [14:49.56] [15:10.64]As they leave the cave [15:12.16] [15:12.28]the stream of bats form a doughnut-shaped ring. [15:16.32] [15:19.40]The wheeling bats seem to confuse a rufus-bellied eagle, [15:23.76] [15:23.88]but they must still survive the attacks of other, more specialised, birds of prey. [15:29.40] [15:40.04]Peregrine falcons and bat hawks are the jetfighters of the bird world. [15:45.72] [16:16.36]Good hunting will end as the light fades [16:19.08] [16:19.20]so the bat hawks bolt their catches on the wing [16:22.92] [16:24.16]and fly straight back for more. [16:26.44] [16:28.80]Any bat separated from the group [16:30.96] [16:31.08]becomes a clear and obvious target and is asking for trouble. [16:35.48] [16:46.44]Yet the nightly onslaught has little impact on bat numbers - [16:50.48] [16:50.60]by the morning the vast majority will be back in the safety of the cave. [16:55.88] [17:05.48]Bats are not the only commuters in these Bornean caves. [17:09.88] [17:10.00]There's a day shift as well. [17:12.08] [17:18.08]Returning from hunting in the sunlight [17:20.32] [17:20.40]these commuters rely on their loud clicks [17:22.84] [17:22.92]to find their way through the cave passages in total darkness. [17:27.24] [17:33.32]They're cave swiftlets. [17:35.44] [17:35.80]Like bats they use echolocation to navigate. [17:39.56] [17:39.68]We need lights to see what's going on, [17:42.16] [17:42.24]but in the pitch black the swiftlets manage unerringly to locate their individual nesting sites, [17:48.00] [17:48.12]which are only a few centimetres across. [17:50.88] [17:55.60]It's a remarkable skill and one we still do not fully understand. [18:01.20] [18:04.84]These birds are unusual for another reason. [18:07.72] [18:07.84]Their little cup-like nests are made entirely from threads of saliva. [18:13.32] [18:18.28]It takes more than 30 days to complete one. [18:21.72] [18:22.92]The nests are very precious objects, [18:25.76] [18:25.76]and not only for the birds. [18:28.40] [18:45.64]For 500 years people have been harvesting the nests of cave swiftlets. [18:51.56] [19:04.00]It's a very risky business. [19:06.48] [19:06.56]with virtually no safety equipment [19:08.76] [19:08.84]and using ladders made from forest vines [19:11.76] [19:11.84]the gatherers climb into the highest reaches of the cave [19:15.20] [19:15.28]often more than 60 metres from the floor. [19:18.28] [19:27.04]The work may be hazardous in the extreme, [19:29.64] [19:29.76]but the rewards are great. [19:32.12] [19:41.64]The pure white nests of cave swiftlets [19:44.28] [19:44.40]are the main ingredient of birds' nest soup [19:47.72] [19:47.84]and gram for gram are worth as much as silver. [19:51.64] [19:53.68]As soon as its nest is removed a bird will immediately build another. [19:58.44] [19:58.56]So, as long as this valuable harvest is properly controlled, [20:02.60] [20:02.68]the colonies will continue to flourish. [20:05.48] [20:17.60]These Bornean caves are among the biggest in the world [20:20.88] [20:20.96]and they're still getting bigger [20:22.36] [20:22.44]as each year rainwater eats away a little more limestone. [20:27.60] [20:37.88]But water in caves doesn't only erode. [20:41.40] [20:41.52]It also builds. [20:43.60] [20:50.32]This water is loaded with dissolved limestone [20:53.96] [20:54.08]and when it meets the air in the cave [20:56.24] [20:56.36]some of that is deposited as a mineral - [20:59.40] [20:59.60]calcite. [21:00.88] [21:03.16]As it builds up [21:04.32] [21:04.40]so the calcite forms decorations that hang from the ceiling - [21:08.48] [21:08.56]stalactites. [21:10.16] [21:17.68]Each drop leaves behind only a miniscule amount of calcite, [21:21.88] [21:22.00]but over time the process can produce some spectacular results. [21:26.88] [21:56.60]If the water seeps though the ceiling quickly, [21:59.32] [21:59.40]then the calcite is deposited on the floor of the cave [22:02.72] [22:02.84]and that creates stalagmites. [22:05.72] [22:14.92]Variations in water flow and the air currents [22:18.08] [22:18.16]produce an infinite variety of forms, but all are created by the same process - [22:23.96] [22:24.28]the slow deposition of dissolved limestone. [22:28.16] [22:31.48]And when stalactite meets stalagmite [22:34.36] [22:34.48]a column is born. [22:36.68] [22:59.84]Structures like these in North America's Carlsbad Cavern [23:03.68] [23:03.76]can take many thousands of years to develop. [23:06.96] [23:07.08]But sometimes the formations in a cave stop growing altogether. [23:12.12] [23:39.44]These flooded caves in Mexico have remained virtually unchanged for thousands of years. [23:46.32] [23:46.40]Since the last Ice Age they've become cut off from the outside world. [23:50.96] [23:51.08]Yet their impact on life on the surface has been huge. [23:55.72] [24:00.96]500 years ago they supported one of the world's great civilisations [24:06.64] [24:06.76]the Maya. [24:07.88] [24:13.04]Mexico's Yucatan Peninsula has no rivers, lakes or streams [24:18.36] [24:18.44]so the Maya relied on the cenotes - the flooded entrances to the water-filled caves. [24:25.08] [24:28.04]These flooded shafts are the region's only source of open fresh water. [24:33.92] [24:36.04]The cenotes are, in effect, gigantic fresh water wells. [24:41.52] [25:05.68]Away from the life-giving rays of sunshine [25:08.64] [25:08.88]one might not expect to find plants. [25:12.20] [25:20.56]But in the darkness of the cave tunnels [25:23.04] [25:23.12]roots of giant tropical trees have pushed their way through cracks in the limestone [25:28.40] [25:28.48]to reach the flooded caverns. [25:30.68] [25:32.36]Without this water [25:33.64] [25:33.76]the Yucatan's forest could not grow so luxuriantly. [25:37.92] [25:57.28]The Maya knew that their lives depended on this water, [26:01.08] [26:01.16]but it's only with the help of today's technology [26:04.00] [26:04.08]that we've come to appreciate the full significance and scale of these flooded passageways. [26:10.80] [26:14.20]So far, more than 350 miles of underwater galleries in the Yucatan have been mapped, [26:21.28] [26:21.40]but still nobody yet knows the true extend of this subterranean waterworld. [26:27.92] [26:28.12]And with good reason. [26:30.28] [26:32.60]Underwater caving is notoriously dangerous. [26:36.76] [26:36.84]When the nearest exit may be hundreds of metres or more away, [26:40.40] [26:40.48]running out of air down here would be fatal. [26:44.08] [26:47.52]To avoid getting lost [26:49.04] [26:49.12]divers carry with them a spool of string. [26:51.84] [26:51.92]It becomes their lifeline - [26:53.76] [26:53.84]literally. [26:54.84] [27:08.36]The string also doubles as a measuring tape - [27:11.48] [27:11.56]a technique that has been used here, in Mexico, [27:14.36] [27:14.44]to chart the largest underwater cave in the world - [27:17.84] [27:17.92]all 100 miles of it. [27:20.52] [27:28.44]Cave exploration often requires you to push yourself through narrow gaps in the rock. [27:34.32] [27:34.56]Cavers call such places 'squeezes.' [27:38.20] [27:47.00]The tighter the squeeze, [27:48.48] [27:48.60]the greater the chance of damaging some vital life-support system. [27:53.60] [28:18.92]In these conditions a diver could easily become disorientated [28:23.84] [28:23.96]and that could be fatal. [28:25.96] [28:39.60]The flooded caverns can play tricks on you in other ways. [28:44.48] [28:52.28]What seems like air, [28:53.84] [28:53.96]isn't. [28:55.12] [28:55.88]It's just another kind of water. [28:58.80] [29:06.16]This is a halocline - [29:08.16] [29:08.24]a meeting of fresh and salt water. [29:11.52] [29:14.08]Fresh water from the jungle flows over the heavier salt water from the sea. [29:19.92] [29:20.92]The saltwater layer is extremely low in oxygen [29:24.36] [29:24.44]making it a particularly difficult place for animals to live. [29:28.36] [29:28.64]Yet some have managed it, [29:30.60] [29:30.76]like the remiped, one of the most ancient of all living crustaceans. [29:35.76] [29:44.00]The Maya understood the importance of the cenotes, [29:47.76] [29:47.88]but they could never have known that these flooded passageways [29:51.20] [29:51.32]were actually the beginning of subterranean rivers, [29:54.92] [29:55.24]all of which eventually flow out to the sea. [29:58.60] [30:08.08]Salt water, unlike fresh water, does not erode limestone, [30:12.40] [30:12.52]so most sea caves are created by the mechanical pounding of the waves. [30:18.24] [30:24.52]The rocky outcrops of New Zealand's Poor Knight Islands are riddled with sea caves [30:30.48] [30:30.56]and just like those in Borneo [30:32.36] [30:32.44]they have become important shelters for many species. [30:35.96] [30:40.44]After a day feeding in the open water [30:43.04] [30:43.12]vast shoals of demoiselle fish return to the caves, [30:47.16] [30:47.24]which they use as a refuge from predators. [30:50.40] [30:52.88]For these fish the caves are a night time retreat, [30:56.60] [30:56.72]but they're not the only commuters in here. [30:59.56] [30:59.64]There are other fish working to a different schedule. [31:03.24] [31:10.36]The bigeyes are the equivalent of bats. [31:13.64] [31:13.76]Night feeders leave the cave each evening. [31:16.88] [31:21.08]And like all cave commuters [31:23.56] [31:23.64]they are most vulnerable at the scheduled time of departure. [31:27.80] [31:38.60]A bottleneck funnels these exiting bats into dense concentrations [31:44.48] [31:44.56]attracting the attention of others. [31:46.96] [32:13.68]The bats can detect the snakes using echolocation, [32:17.80] [32:18.04]but the snakes are literally in the dark - [32:20.60] [32:20.68]they can see nothing. [32:22.48] [32:35.64]The strikes seem to be largely hit-and-miss, [32:39.24] [32:41.08]but the snakes have a secret weapon. [32:44.16] [32:44.52]They can actually sense each bat flying past. [32:48.00] [32:48.12]Receptors in the snake's head pick up the heat given off by the flying bats, [32:53.48] [32:53.72]as this thermal image shows. [32:56.12] [33:02.52]To the snakes the bats are apparently glowing [33:06.16] [33:06.24]and this gives them something to aim at. [33:09.08] [33:30.60]This is the price that these cave commuters must pay [33:34.52] [33:34.64]for their daytime sanctuary on the ground. [33:37.72] [33:37.92]Small wonder then that there are other cave dwellers that stay put. [33:43.08] [33:50.36]Many caves are like islands - [33:52.72] [33:52.80]cut off from the outside world and from other caves. [33:57.08] [34:04.72]This isolation has resulted in the evolution of some various strange creatures. [34:11.44] [34:13.20]They are the cave specialists - [34:15.88] [34:15.96]troglodytes, animals that never emerge from the caves or see daylight. [34:21.72] [34:34.12]These troglodytes from Thailand [34:36.72] [34:36.80]are possibly the most specialised creatures on Earth [34:40.44] [34:40.56]for they live only in cave waterfalls. [34:44.04] [34:44.84]The entire population of these cave angel fish [34:48.12] [34:48.24]seems to be restricted to just two small caves. [34:52.44] [34:56.24]It's the same story with other troglodytes. [34:58.80] [34:58.88]There may well be less than a hundred Texas cave salamanders in the wild. [35:04.16] [35:20.16]And the Belizean white crab is another creature [35:24.00] [35:24.12]that is unique to just one cave system. [35:27.60] [35:32.28]Living in perpetual darkness [35:34.32] [35:34.44]they have all not only lost the pigment in their skin, [35:37.44] [35:37.60]but also their eyes. [35:39.48] [35:41.48]It takes thousands of generations for eyes to be lost, [35:45.16] [35:45.24]so these species must have been isolated for a very long time. [35:50.16] [35:54.84]But the blind salamander has other highly developed sensory organs. [36:00.20] [36:02.68]Receptors in their skin detect minute movements in the water made by its prey. [36:08.64] [36:14.84]External gills help it to breathe in water that is particularly low in oxygen. [36:20.56] [36:31.84]The cave angel fish feed on bacteria in the fast flowing water [36:36.76] [36:36.84]keeping their grip with microscopic hooks on their fins. [36:41.16] [36:51.36]Food is often in short supply [36:53.80] [36:53.96]and troglodytes like the crab [36:56.20] [36:56.32]have to survive on whatever washes into the cave from outside. [37:00.24] [37:04.52]The salamander might not encounter food for several months, [37:08.20] [37:08.32]so when something does come along [37:10.20] [37:10.36]it can't afford to miss it. [37:12.28] [37:19.04]It's astonishing that these extraordinary cave dwellers manage to survive at all. [37:25.20] [37:25.64]But one cave is so inhospitable [37:28.08] [37:28.20]that one would not expect it to contain any life whatsoever. [37:31.96] [37:37.72]The water flowing out of the Villa Luz cave in Mexico [37:41.68] [37:41.80]is actually coloured white with sulphuric acid. [37:45.72] [38:00.08]Explorers entering this dangerous cave [38:02.88] [38:03.00]must wear respirators and carry monitors. [38:06.64] [38:06.72]Poisonous gases rise to fatal levels so quickly [38:10.04] [38:10.16]that an early warning system is essential. [38:13.08] [38:17.72]Bats survive by staying close to the skylights, [38:21.36] [38:21.48]but venturing deep into the cave is very dangerous indeed. [38:26.12] [38:35.24]The source of these toxic fumes lies several miles below. [38:40.72] [38:40.92]Hydrogen sulphide gas bubbles up from oil deposits in the earth's crust. [38:46.04] [38:46.16]It mixes with oxygen and the water, [38:48.48] [38:48.60]and forms sulphuric acid. [38:51.16] [39:02.04]These are not the sort of conditions in which you would expect to find fish, [39:06.28] [39:06.40]yet these cave mollies seem to thrive [39:09.32] [39:09.44]despite the acid and the low levels of oxygen. [39:12.76] [39:15.28]There is, in fact, more life here than anyone would think possible, [39:19.20] [39:19.32]but the biggest surprise is something altogether more bizarre. [39:24.28] [39:34.12]These strange stalactite-like formations are known, rather appropriately, as snotites, [39:42.44] [39:42.56]the drops dripping from the ends are sulphuric acid, [39:46.80] [39:46.92]strong enough to burn skin. [39:49.32] [39:52.76]The snotites are, in fact, vast colonies of bacteria, [39:56.84] [39:56.96]capable of going a centimetre a day. [40:00.28] [40:04.52]In this world without sunlight [40:06.48] [40:06.60]these bacteria extract energy from the hydrogen sulphide gas. [40:11.64] [40:16.04]Bacteria like these are known as extremofile [40:19.92] [40:20.04]because of their ability to survive in such extreme conditions. [40:24.56] [40:26.92]And these extremofiles play another important role in this cave. [40:31.72] [40:31.84]Surprisingly, they are the basis of a food chain [40:35.32] [40:35.40]which supports, amongst other creatures, the larvae of these midges. [40:40.00] [40:49.40]Villa Luz's ecosystem was certainly very remarkable, [40:52.96] [40:53.12]but cave explorers were soon to make an even more astonishing discovery. [40:58.44] [41:10.64]Beneath this arid landscape lies a subterranean wonderland. [41:15.84] [41:21.60]Without water one might not expect to find any caves, [41:25.16] [41:25.28]but beneath these rolling desert slopes in the United States [41:29.48] [41:29.60]lies one of the longest, deepest and most surprising caves in the world. [41:36.28] [41:40.08]Its secrets remained unknown until 1986, [41:44.16] [41:44.28]when cavers dug through several metres of loose rock [41:47.68] [41:47.76]to the bottom of this pit. [41:49.76] [41:53.08]They named the cave 'Lechuguilla' [41:55.88] [41:56.04]and since this discovery more than 120 miles of passageways have been mapped. [42:03.04] [42:08.00]When the first explorers descended, [42:10.08] [42:10.20]no-one guessed at the sheer size of this cave. [42:13.52] [42:13.64]But even that was not going to be the biggest surprise. [42:17.56] [42:17.72]Little did they realise that Lechuguilla [42:20.00] [42:20.12]would soon be regarded by cavers the world over [42:23.00] [42:23.12]as the most beautiful of all caves. [42:26.60] [42:26.88]They were about to discover [42:28.20] [42:28.32]some of the most exquisite formations ever seen underground. [42:33.40] [42:47.12]The walls were covered with the most delicate and fragile crystals. [42:52.44] [43:07.04]Many of these crystals were made of gypsum, [43:10.24] [43:10.36]a mineral that comes from limestone. [43:12.88] [43:13.16]And there was mile after mile of them. [43:16.32] [43:33.04]Water is the creator of most caves, [43:36.32] [43:36.40]but, unlike all other limestone caves, [43:39.08] [43:39.20]Lechuguilla's rock had not been eaten away by running rainwater. [43:44.28] [43:45.64]Something else was responsible. [43:48.00] [43:56.08]The only water Lechuguilla has are these wonderfully still clear pools. [44:02.72] [44:07.36]As the explorers went deeper into the cave, [44:09.96] [44:10.08]they came across whole galleries filled with the most unusual formations, [44:14.80] [44:14.92]like these 5-metre cones, frosted with the most delicate crystals. [44:20.24] [44:48.88]It was Lechuguilla's gypsum crystals [44:51.60] [44:51.72]that made scientists question how these caverns were formed. [44:55.84] [44:58.08]They discovered that Lechuguilla's limestone had actually been eaten away by sulphuric acid, [45:05.00] [45:05.12]cutting through literally miles of limestone. [45:08.72] [45:25.60]And when sulphuric acid dissolves limestone [45:28.64] [45:28.76]it leaves behind gypsum, [45:30.92] [45:31.04]the basis of Lechuguilla's remarkable formations. [45:34.68] [45:34.80]And there was one set, [45:36.24] [45:36.36]more than a mile from the surface, [45:38.68] [45:38.76]that almost defied belief. [45:41.28] [45:57.56]The Chandelier Ballroom was the ultimate discovery. [46:01.84] [46:01.96]With its six-metre long crystals [46:04.32] [46:04.44]it's surely the most bizarre cave chamber in the world. [46:09.96] [46:49.68]And the walls had one further surprise. [46:54.44] [46:55.96]Extremofile bacteria were found to be feeding on the rock itself. [47:01.96] [47:07.64]The discovery of life that exists without drawing any of its energy from the sun [47:13.00] [47:13.12]shows us once again how complex and surprising the underground world can be. [47:19.64] [47:23.20]Each year explorers chart over a hundred miles of new cave passages. [47:29.04] [47:30.84]But with half the world's limestone still to be explored, [47:34.76] [47:34.88]who knows how many Lechuguillas are still waiting to be discovered? [47:40.64]

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