0:00:39 > 0:00:44This is our planet's final frontier -
0:00:46 > 0:00:51an inner world, where only the most adventurous dare to go.
0:01:43 > 0:01:48Beneath our feet are countless miles of cave shafts and passages.
0:02:05 > 0:02:07The Cave of Swallows in Mexico.
0:02:07 > 0:02:09400 metres to the bottom -
0:02:09 > 0:02:14deep enough to engulf the Empire State Building.
0:02:21 > 0:02:24This is the biggest cave shaft in the world,
0:02:24 > 0:02:26yet these depths were first explored
0:02:26 > 0:02:30only two years before men landed on the moon.
0:02:33 > 0:02:39Today, caves remain the least explored places on Earth.
0:02:39 > 0:02:42However, human beings are seldom the first
0:02:42 > 0:02:45to reach these black, damp places.
0:02:50 > 0:02:56Here live some of the strangest and least-known animals on the planet.
0:03:28 > 0:03:31This galaxy of little lights
0:03:31 > 0:03:34is created by thousands of living creatures.
0:03:39 > 0:03:44Any animal that lives in a cave has to cope with complete blackness.
0:03:44 > 0:03:50But in New Zealand, some have turned this darkness to their advantage.
0:04:02 > 0:04:06A silken strand is lowered from the ceiling,
0:04:06 > 0:04:08alongside hundreds of others.
0:04:16 > 0:04:21Beautiful though these threads are, they have a sinister purpose.
0:04:22 > 0:04:26This is a cave glow-worm.
0:04:26 > 0:04:31To trap its prey, it goes fishing with a line of silk
0:04:37 > 0:04:42The silk comes from glands in the glow-worm's mouth
0:04:42 > 0:04:45and is loaded with droplets of mucus.
0:04:59 > 0:05:04Each glow-worm produces dozens of these threads
0:05:06 > 0:05:08Once its lines are set,
0:05:08 > 0:05:11the glow-worm hangs from a mucus hammock
0:05:11 > 0:05:14and waits, like a patient angler.
0:05:18 > 0:05:22But the glow-worm doesn't leave everything to chance.
0:05:22 > 0:05:24That ghostly blue light
0:05:24 > 0:05:27is the result of a chemical reaction
0:05:27 > 0:05:30taking place inside a special capsule in its tail.
0:05:30 > 0:05:34The light literally shines out of its backside.
0:05:34 > 0:05:38It's a lure for attracting prey.
0:06:04 > 0:06:08Insects seem irresistibly drawn towards the source
0:06:08 > 0:06:12and then get trapped by the sticky lines.
0:06:21 > 0:06:24Once stuck, there is no escape.
0:06:29 > 0:06:34Now it's just a matter of reeling in the line
0:06:34 > 0:06:37and slowly consuming the catch alive.
0:06:52 > 0:06:56By ensnaring the insects that hatch in this cave,
0:06:56 > 0:06:59these glow-worms have solved the biggest challenge
0:06:59 > 0:07:01that permanent cave-dwellers face -
0:07:01 > 0:07:04finding a regular and reliable source of food.
0:07:12 > 0:07:17One kind of rock makes this whole underground world possible -
0:07:17 > 0:07:18limestone.
0:07:18 > 0:07:21Most of the world's caves are found within it
0:07:21 > 0:07:25and it covers nearly 10% of the Earth's surface.
0:07:31 > 0:07:36Limestone is composed of minerals derived from shells and corals.
0:07:36 > 0:07:40Although this rocky escarpment in the United States
0:07:40 > 0:07:43is now hundreds of metres above sea level,
0:07:43 > 0:07:45it was actually formed underwater.
0:07:52 > 0:07:56The limestone towers of Vietnam's Ha Long Bay
0:07:56 > 0:07:59are a reminder of this link with the sea.
0:07:59 > 0:08:04Originally this whole area would have been one solid block of limestone -
0:08:04 > 0:08:06the base of a coral reef.
0:08:16 > 0:08:20In Borneo, rain has sculpted the limestone
0:08:20 > 0:08:23into extremely sharp-sided pinnacles.
0:08:37 > 0:08:40But the dissolving power of rainwater has other,
0:08:40 > 0:08:44much more dramatic, effects underground.
0:08:55 > 0:08:59Rivers that flow over limestone often seem to completely disappear.
0:09:14 > 0:09:17When the water reaches a more resistant bed of limestone,
0:09:17 > 0:09:19its course is altered.
0:09:23 > 0:09:28Once underground the water takes on a new, more erosive power.
0:09:35 > 0:09:37During its journey from the surface,
0:09:37 > 0:09:40the water absorbs carbon dioxide from the soil,
0:09:40 > 0:09:43making it mildly acidic,
0:09:43 > 0:09:47and over millions of years this acid eats away the limestone,
0:09:47 > 0:09:52creating a maze of caverns and passages that can go on for miles.
0:10:31 > 0:10:36This is the biggest underground river passage in the world.
0:10:36 > 0:10:41So big, a jumbo jet could fly through it.
0:10:41 > 0:10:43It's Deer Cave in Borneo.
0:10:55 > 0:10:59The sheer size of Deer Cave allows some animals
0:10:59 > 0:11:02to gather there in huge numbers.
0:11:09 > 0:11:14A staggering three million wrinkle-lipped bats live here.
0:11:17 > 0:11:21The bats roost high on the walls and ceilings,
0:11:21 > 0:11:24where they're well protected from the elements
0:11:24 > 0:11:26and safe from predators.
0:11:33 > 0:11:38And while they're up there, the bats produce something very important.
0:11:42 > 0:11:48This 100-metre-high mound is made entirely of bat droppings -
0:11:48 > 0:11:49guano.
0:12:11 > 0:12:16Its surface is covered by a thick carpet of cockroaches -
0:12:16 > 0:12:19hundreds of thousands of them.
0:12:24 > 0:12:28Caves are one of the few habitats on Earth
0:12:28 > 0:12:30not directly powered by sunlight.
0:12:30 > 0:12:35In the absence of plants, this food chain is based
0:12:35 > 0:12:38on a continuous supply of bat droppings.
0:12:50 > 0:12:54The cockroaches feed on the guano
0:12:54 > 0:12:56and anything that falls into it.
0:13:23 > 0:13:27The droppings also support other types of cockroaches
0:13:27 > 0:13:30which spend part of their day resting on cave walls.
0:13:35 > 0:13:39These, in turn, become food for giant cave centipedes,
0:13:39 > 0:13:42some more than 20 centimetres long.
0:13:45 > 0:13:48Bizarrely, there are crabs here too,
0:13:48 > 0:13:51sifting through the droppings for nutrients.
0:13:58 > 0:14:02All these animals spend their entire lives within the cave.
0:14:02 > 0:14:06They're totally dependent on the digested remains of food
0:14:06 > 0:14:08that's brought in from outside.
0:14:35 > 0:14:37Each evening, in just two hours,
0:14:37 > 0:14:41three million bats leave the safety of the cave
0:14:41 > 0:14:45to hunt for insects in the forest outside.
0:14:46 > 0:14:49But not all will return.
0:15:10 > 0:15:16As they leave the cave, the stream of bats form a doughnut-shaped ring.
0:15:19 > 0:15:23The wheeling bats seem to confuse a rufous-bellied eagle
0:15:23 > 0:15:26but they must still survive the attacks of other,
0:15:26 > 0:15:28more specialised, birds of prey.
0:15:40 > 0:15:44Peregrine falcons and bat hawks are the jet fighters of the bird world.
0:16:16 > 0:16:19Good hunting will end as the light fades,
0:16:19 > 0:16:22so the bat hawks bolt their catches on the wing...
0:16:24 > 0:16:26..and fly straight back for more.
0:16:28 > 0:16:33Any bat separated from the group becomes a clear and obvious target
0:16:33 > 0:16:35and is asking for trouble.
0:16:46 > 0:16:50Yet the nightly onslaught has little impact on bat numbers.
0:16:50 > 0:16:52By the morning,
0:16:52 > 0:16:55the vast majority will be back in the safety of the cave.
0:17:05 > 0:17:09Bats are not the only commuters in these Bornean caves.
0:17:09 > 0:17:12There's a day shift as well.
0:17:18 > 0:17:21Returning from hunting in the sunlight,
0:17:21 > 0:17:23these commuters rely on their loud clicks
0:17:23 > 0:17:27to find their way through the cave passages in total darkness.
0:17:33 > 0:17:35They are cave swiftlets.
0:17:35 > 0:17:39Like bats, they use echolocation to navigate.
0:17:39 > 0:17:42We need lights to see what's going on,
0:17:42 > 0:17:46but in the pitch black the swiftlets manage unerringly
0:17:46 > 0:17:48to locate their individual nesting sites
0:17:48 > 0:17:51which are only a few centimetres across.
0:17:55 > 0:18:00It's a remarkable skill and one we still do not fully understand.
0:18:04 > 0:18:07These birds are unusual for another reason.
0:18:07 > 0:18:12Their little cup-like nests are made entirely from threads of saliva.
0:18:18 > 0:18:22It takes more than 30 days to complete one.
0:18:22 > 0:18:25Their nests are very precious objects
0:18:25 > 0:18:27and not only for the birds.
0:18:29 > 0:18:32MEN CHANT RHYTHMICALLY
0:18:45 > 0:18:47For 500 years,
0:18:47 > 0:18:51people have been harvesting the nests of cave swiftlets.
0:19:04 > 0:19:06It's a very risky business.
0:19:06 > 0:19:08With virtually no safety equipment,
0:19:08 > 0:19:11and using ladders made from forest vines,
0:19:11 > 0:19:15the gatherers climb into the highest reaches of the cave,
0:19:15 > 0:19:18often more than 60 metres from the floor.
0:19:27 > 0:19:32The work may be hazardous in the extreme but the rewards are great.
0:19:41 > 0:19:45The pure white nests of cave swiftlets
0:19:45 > 0:19:47are the main ingredient of bird's nest soup
0:19:47 > 0:19:52and, gram for gram, are worth as much as silver.
0:19:53 > 0:19:58As soon as its nest is removed, a bird will immediately build another.
0:19:58 > 0:20:03So as long as this valuable harvest is properly controlled,
0:20:03 > 0:20:05the colonies will continue to flourish.
0:20:17 > 0:20:21These Bornean caves are among the biggest in the world
0:20:21 > 0:20:24and they're still getting bigger as, each year,
0:20:24 > 0:20:27rainwater eats away a little more limestone.
0:20:37 > 0:20:43But water in caves doesn't only erode, it also builds.
0:20:50 > 0:20:53This water is loaded with dissolved limestone
0:20:53 > 0:20:56and when it meets the air in the cave,
0:20:56 > 0:21:00some of that is deposited as a mineral - calcite.
0:21:02 > 0:21:04As it builds up,
0:21:04 > 0:21:09so the calcite forms decorations that hang from the ceiling - stalactites.
0:21:17 > 0:21:22Each drop leaves behind only a minuscule amount of calcite
0:21:22 > 0:21:26but over time the process can produce some spectacular results.
0:21:56 > 0:21:59If the water seeps through the ceiling quickly,
0:21:59 > 0:22:02then the calcite is deposited on the floor...
0:22:02 > 0:22:05and that creates stalagmites.
0:22:14 > 0:22:18Variations in water flow and air currents
0:22:18 > 0:22:20produce an infinite variety of forms
0:22:20 > 0:22:23but all are created by the same process -
0:22:23 > 0:22:27the slow deposition of dissolved limestone.
0:22:31 > 0:22:36And when stalactite meets stalagmite, a column is born.
0:22:59 > 0:23:03Structures like these in North America's Carlsbad Cavern
0:23:03 > 0:23:07can take many thousands of years to develop.
0:23:07 > 0:23:11But sometimes the formations in a cave stop growing altogether.
0:23:39 > 0:23:41These flooded caves in Mexico
0:23:41 > 0:23:46have remained virtually unchanged for thousands of years.
0:23:46 > 0:23:48Since the last ice age,
0:23:48 > 0:23:51they have become cut off from the outside world,
0:23:51 > 0:23:55yet their impact on life at the surface has been huge.
0:24:00 > 0:24:02Five hundred years ago,
0:24:02 > 0:24:06they supported one of the world's great civilisations -
0:24:06 > 0:24:08the Maya.
0:24:13 > 0:24:18Mexico's Yucatan peninsula has no rivers, lakes or streams,
0:24:18 > 0:24:20so the Maya relied on the cenotes -
0:24:20 > 0:24:24the flooded entrances to the water-filled caves.
0:24:27 > 0:24:33These flooded shafts are the region's only source of open fresh water.
0:24:35 > 0:24:41The cenotes are, in effect, gigantic freshwater wells.
0:25:05 > 0:25:08Away from the life-giving rays of sunshine,
0:25:08 > 0:25:12one might not expect to find plants.
0:25:20 > 0:25:22But in the darkness of the cave tunnels,
0:25:22 > 0:25:26roots of giant tropical trees have pushed their way
0:25:26 > 0:25:29through cracks in the limestone to reach the flooded caverns.
0:25:32 > 0:25:37Without this water, the Yucatan's forest could not grow so luxuriantly.
0:25:57 > 0:26:00The Maya knew that their lives depended on this water
0:26:00 > 0:26:04but it's only with the help of today's technology
0:26:04 > 0:26:07that we've come to appreciate the full significance
0:26:07 > 0:26:10and scale of these flooded passageways.
0:26:14 > 0:26:17So far, more than 350 miles
0:26:17 > 0:26:22of underwater galleries in the Yucatan have been mapped
0:26:22 > 0:26:24but still nobody yet knows
0:26:24 > 0:26:28the true extent of this subterranean water world.
0:26:28 > 0:26:30And with good reason.
0:26:32 > 0:26:36Underwater caving is notoriously dangerous.
0:26:36 > 0:26:40When the nearest exit may be hundreds of metres or more away,
0:26:40 > 0:26:43running out of air down here would be fatal.
0:26:47 > 0:26:52To avoid getting lost, divers carry with them a spool of string.
0:26:52 > 0:26:54It becomes their lifeline... literally.
0:27:08 > 0:27:11The string also doubles as a measuring tape -
0:27:11 > 0:27:14a technique that has been used, here in Mexico,
0:27:14 > 0:27:17to chart the largest underwater cave in the world -
0:27:17 > 0:27:20all 100 miles of it.
0:27:28 > 0:27:31Cave exploration often requires you
0:27:31 > 0:27:34to push yourself through narrow gaps in the rock.
0:27:34 > 0:27:37Cavers call such places squeezes.
0:27:47 > 0:27:49The tighter the squeeze,
0:27:49 > 0:27:53the greater the chance of damaging some vital life-support system.
0:28:18 > 0:28:24In these conditions, a diver could easily become disorientated
0:28:24 > 0:28:26and that could be fatal.
0:28:38 > 0:28:44The flooded caverns can play tricks on you in other ways.
0:28:52 > 0:28:54What seems like air...isn't.
0:28:55 > 0:28:58It's just another kind of water.
0:29:05 > 0:29:11This is a halocline - a meeting of fresh and salt water.
0:29:14 > 0:29:16Fresh water from the jungle
0:29:16 > 0:29:21flows over the heavier salt water from the sea.
0:29:21 > 0:29:24The salt water layer is extremely low in oxygen,
0:29:24 > 0:29:28making it a particularly difficult place for animals to live.
0:29:28 > 0:29:31Yet some have managed it, like the remipede,
0:29:31 > 0:29:35one of the most ancient of all living crustaceans.
0:29:43 > 0:29:47The Maya understood the importance of the cenotes
0:29:47 > 0:29:51but they could never have known that these flooded passageways
0:29:51 > 0:29:54were actually the beginning of subterranean rivers,
0:29:54 > 0:29:58all of which eventually flow out to the sea.
0:30:07 > 0:30:12Salt water, unlike fresh water, does not erode limestone.
0:30:12 > 0:30:18So most sea caves are created by the mechanical pounding of waves.
0:30:23 > 0:30:28The rocky outcrops of New Zealand's Poor Knights Islands
0:30:28 > 0:30:32are riddled with sea caves and, just like those in Borneo,
0:30:32 > 0:30:35they have become important shelters for many species.
0:30:40 > 0:30:44After a day feeding in the open water,
0:30:44 > 0:30:47vast shoals of demoiselle fish return to the caves,
0:30:47 > 0:30:50which they use as a refuge from predators.
0:30:53 > 0:30:57For these fish, the caves are a night-time retreat
0:30:57 > 0:30:59but they are not the only commuters in here.
0:30:59 > 0:31:03There are other fish, working to a different schedule.
0:31:10 > 0:31:13The Big Eyes are the equivalent of bats...
0:31:13 > 0:31:16night feeders that leave the cave each evening.
0:31:20 > 0:31:25And like all cave commuters, they are most vulnerable
0:31:25 > 0:31:27at the scheduled time of departure.
0:31:38 > 0:31:44A bottle-neck funnels the exiting bats into dense concentrations,
0:31:44 > 0:31:46attracting the attention of others.
0:32:13 > 0:32:18The bats can detect the snakes using echolocation,
0:32:18 > 0:32:22but the snakes are literally in the dark. They can see nothing.
0:32:35 > 0:32:39The strikes seem to be largely hit-and-miss...
0:32:41 > 0:32:44..but the snakes have a secret weapon.
0:32:44 > 0:32:48They can actually sense each bat flying past.
0:32:48 > 0:32:50Receptors in the snake's head
0:32:50 > 0:32:53pick up the heat given off by the flying bats,
0:32:53 > 0:32:56as this thermal image shows.
0:33:02 > 0:33:06To the snakes, the bats are apparently glowing
0:33:06 > 0:33:09and this gives them something to aim at.
0:33:30 > 0:33:33This is the price that these cave commuters
0:33:33 > 0:33:37must pay for their daytime sanctuary underground.
0:33:37 > 0:33:43Small wonder, then, that there are other cave dwellers that stay put.
0:33:50 > 0:33:52Many caves are like islands,
0:33:52 > 0:33:56cut off from the outside world and from other caves.
0:34:04 > 0:34:06This isolation
0:34:06 > 0:34:10has resulted in the evolution of some very strange creatures.
0:34:12 > 0:34:15They are the cave specialists.
0:34:15 > 0:34:19Troglodytes - animals that never emerge
0:34:19 > 0:34:21from the caves or see daylight.
0:34:34 > 0:34:37These troglodytes from Thailand
0:34:37 > 0:34:41are possibly the most specialised creatures on Earth,
0:34:41 > 0:34:44for they live only in cave waterfalls.
0:34:44 > 0:34:49The entire population of these cave angelfish seems to be
0:34:49 > 0:34:52restricted to just two small caves.
0:34:56 > 0:34:58It's the same story with other troglodytes.
0:34:58 > 0:35:03There may be less than 100 Texas cave salamanders in the wild.
0:35:19 > 0:35:25And the Belizean white crab is another creature that is unique
0:35:25 > 0:35:27to just one cave system.
0:35:31 > 0:35:36Living in perpetual darkness, they have all not only lost the pigment
0:35:36 > 0:35:39in their skin, but also their eyes.
0:35:41 > 0:35:44It takes thousands of generations for eyes to be lost,
0:35:44 > 0:35:48so these species must have been isolated for a very long time.
0:35:54 > 0:35:59But the blind salamander has other highly developed sensory organs.
0:36:02 > 0:36:06Receptors in the skin detect minute movements in the water
0:36:06 > 0:36:08made by its prey.
0:36:14 > 0:36:17External gills help it to breathe in water
0:36:17 > 0:36:20that is particularly low in oxygen.
0:36:31 > 0:36:36The cave angelfish feed on bacteria in the fast-flowing water,
0:36:36 > 0:36:40keeping their grip with microscopic hooks on their fins.
0:36:51 > 0:36:56Food is often in short supply and troglodytes like the crab
0:36:56 > 0:37:00have to survive on whatever washes into the cave from outside.
0:37:04 > 0:37:08A salamander might not encounter food for several months,
0:37:08 > 0:37:12so when something does come along, it can't afford to miss it.
0:37:18 > 0:37:20It is astonishing
0:37:20 > 0:37:24that these extraordinary cave dwellers manage to survive at all,
0:37:24 > 0:37:28but one cave is so inhospitable
0:37:28 > 0:37:31that one would not expect it to contain any life whatsoever.
0:37:37 > 0:37:41The water flowing out of the Villa Luz Cave in Mexico
0:37:41 > 0:37:45is actually coloured white with sulphuric acid.
0:37:59 > 0:38:03Explorers entering this dangerous cave
0:38:03 > 0:38:06must wear respirators and carry monitors.
0:38:06 > 0:38:10Poisonous gases rise to fatal levels so quickly
0:38:10 > 0:38:13that an early warning system is essential.
0:38:17 > 0:38:21Bats survive by staying close to the skylights,
0:38:21 > 0:38:25but venturing deep into the cave is very dangerous indeed.
0:38:34 > 0:38:40The source of the toxic fumes lies several miles below.
0:38:40 > 0:38:46Hydrogen sulphide gas bubbles up from oil deposits in the Earth's crust.
0:38:46 > 0:38:51It mixes with oxygen in the water and forms sulphuric acid.
0:39:02 > 0:39:06These are not the sort of conditions in which you'd expect to find fish.
0:39:06 > 0:39:10Yet these cave mollies seem to thrive, despite the acid
0:39:10 > 0:39:12and the low levels of oxygen.
0:39:15 > 0:39:19There is, in fact, more life here than anyone would think possible,
0:39:19 > 0:39:24but the biggest surprise is something altogether more bizarre.
0:39:34 > 0:39:38These strange stalactite-like formations
0:39:38 > 0:39:42are known, rather appropriately, as "snottites".
0:39:42 > 0:39:45The drops dripping from the ends
0:39:45 > 0:39:49are sulphuric acid, strong enough to burn skin.
0:39:52 > 0:39:56The snottites are, in fact, vast colonies of bacteria,
0:39:56 > 0:40:00capable of growing a centimetre a day.
0:40:04 > 0:40:06In this world without sunlight,
0:40:06 > 0:40:11these bacteria extract energy from the hydrogen sulphide gas.
0:40:15 > 0:40:20Bacteria like these are known as "extremophiles"
0:40:20 > 0:40:24because of their ability to survive in such extreme conditions.
0:40:26 > 0:40:31And these extremophiles play another important role in this cave.
0:40:31 > 0:40:36Surprisingly, they are the basis of a food chain which supports,
0:40:36 > 0:40:39amongst other creatures, the larvae of these midges.
0:40:49 > 0:40:53Villa Luz's ecosystem was certainly very remarkable,
0:40:53 > 0:40:58but cave explorers were soon to make an even more astonishing discovery.
0:41:10 > 0:41:15Beneath this arid landscape lies a subterranean wonderland.
0:41:21 > 0:41:25Without water, one might not expect to find any caves,
0:41:25 > 0:41:29but beneath these rolling desert slopes in the United States
0:41:29 > 0:41:34lies one of the longest, deepest and most surprising caves in the world.
0:41:39 > 0:41:43Its secrets remained unknown until 1986,
0:41:43 > 0:41:47when cavers dug through several metres of loose rock
0:41:47 > 0:41:49at the bottom of this pit.
0:41:52 > 0:41:55They named the cave Lechuguilla
0:41:55 > 0:41:58and since its discovery
0:41:58 > 0:42:02more than 120 miles of passageways have been mapped.
0:42:07 > 0:42:10When the first explorers descended,
0:42:10 > 0:42:13no-one guessed at the sheer size of this cave.
0:42:13 > 0:42:17But even that was not going to be the biggest surprise.
0:42:17 > 0:42:20Little did they realise that Lechuguilla would soon
0:42:20 > 0:42:26be regarded by cavers the world over as the most beautiful of all caves.
0:42:26 > 0:42:31They were about to discover some of the most exquisite formations
0:42:31 > 0:42:32ever seen underground.
0:42:47 > 0:42:51The walls were covered with the most delicate and fragile crystals.
0:43:06 > 0:43:10Many of these crystals were made of gypsum,
0:43:10 > 0:43:13a mineral that comes from limestone,
0:43:13 > 0:43:15and there was mile after mile of them.
0:43:32 > 0:43:36Water is the creator of most caves,
0:43:36 > 0:43:39but unlike all other limestone caves,
0:43:39 > 0:43:44Lechuguilla's rock had not been eaten away by running rainwater.
0:43:45 > 0:43:47Something else was responsible.
0:43:55 > 0:44:01The only water Lechuguilla has are these wonderfully still, clear pools.
0:44:07 > 0:44:10As the explorers went deeper into the cave,
0:44:10 > 0:44:14they came across whole galleries filled with unusual formations,
0:44:14 > 0:44:19like these 5-metre cones frosted with the most delicate crystals.
0:44:48 > 0:44:51It was Lechuguilla's gypsum crystals
0:44:51 > 0:44:57that made scientists question how these caverns were formed.
0:44:57 > 0:45:00They discovered that Lechuguilla's limestone
0:45:00 > 0:45:03had actually been eaten away
0:45:03 > 0:45:08by sulphuric acid, cutting through literally miles of limestone.
0:45:25 > 0:45:30And when sulphuric acid dissolves limestone, it leaves behind gypsum -
0:45:30 > 0:45:34the basis of Lechuguilla's remarkable formations.
0:45:34 > 0:45:38And there was one set, more than a mile from the surface,
0:45:38 > 0:45:40that almost defied belief.
0:45:57 > 0:46:01The Chandelier Ballroom was the ultimate discovery.
0:46:01 > 0:46:04With its 6-metre-long crystals,
0:46:04 > 0:46:08it's surely the most bizarre cave chamber in the world.
0:46:49 > 0:46:52And the walls had one further surprise.
0:46:56 > 0:47:01Extremophile bacteria were found to be feeding on the rock itself.
0:47:07 > 0:47:11The discovery of life that exists without drawing any of its energy
0:47:11 > 0:47:14from the sun shows us once again
0:47:14 > 0:47:19how complex and surprising the underground world can be.
0:47:23 > 0:47:28Each year, explorers chart over 100 miles of new cave passages.
0:47:30 > 0:47:34But with half the world's limestone still to be explored,
0:47:34 > 0:47:40who knows how many Lechuguillas are still waiting to be discovered?
0:48:04 > 0:48:09Of all the habitats the Planet Earth teams had to deal with, undoubtedly,
0:48:09 > 0:48:14the one that was to provide the most unpleasant working conditions
0:48:14 > 0:48:17was the underground world of caves.
0:48:17 > 0:48:19Look at that!
0:48:19 > 0:48:22Thousands of cockroaches!
0:48:22 > 0:48:26There are a few juveniles in the moulting phase here...
0:48:26 > 0:48:29Just everywhere you look is cockroaches.
0:48:29 > 0:48:31Look at that.
0:48:31 > 0:48:32Beautiful sight.
0:48:37 > 0:48:41Gomantong Cave is home to the world's largest concentrations
0:48:41 > 0:48:46of cockroaches. And the team are going to have to spend a month
0:48:46 > 0:48:48working in this massive pile of guano.
0:48:48 > 0:48:52And we're back here with the biggest mound of doo-doo
0:48:52 > 0:48:55you've ever seen in the world.
0:48:55 > 0:48:57And, uh...it's a bit grim.
0:48:57 > 0:49:01We're going to get dressed up in all the stuff.
0:49:01 > 0:49:04You get cockroaches in your neck and down your pants...
0:49:04 > 0:49:07Oh, God!
0:49:07 > 0:49:09Do we have to go back up?
0:49:09 > 0:49:13The crew's goal here was to try and convey the sheer scale
0:49:13 > 0:49:16of this mound of droppings.
0:49:16 > 0:49:22The aim was a continuous smooth shot from the base to the very summit.
0:49:22 > 0:49:27Now, I'm actually taping up the gusset of my paper suit here,
0:49:27 > 0:49:30because I've spent the last few days with it splitting.
0:49:30 > 0:49:34I'd be sitting down waist-deep in faeces, and then you can just feel
0:49:34 > 0:49:37the insects crawling in.
0:49:37 > 0:49:41And it's just not an area where you want insects!
0:49:43 > 0:49:47Every inch of the ground was covered with cockroaches.
0:49:47 > 0:49:53As soon as you put a lens or camera down, it was engulfed by bugs.
0:49:53 > 0:49:57Death and decay was everywhere.
0:50:00 > 0:50:03These guys are supposed to be up on the cave roof above us,
0:50:03 > 0:50:06but I don't think this one has any idea where he is.
0:50:11 > 0:50:15This tracking system was a bit Heath Robinson, and keeping it working
0:50:15 > 0:50:19in this filthy, damp environment was extremely difficult.
0:50:20 > 0:50:22Oh, no!
0:50:22 > 0:50:25We've just come off the wheel.
0:50:25 > 0:50:28So many things go wrong in here with equipment -
0:50:28 > 0:50:30it's so humid and so messy
0:50:30 > 0:50:35that everything gets gunk in it and fuses short-circuit.
0:50:35 > 0:50:36Terrible.
0:50:36 > 0:50:39Absolutely terrible.
0:50:39 > 0:50:40Can't use it.
0:50:44 > 0:50:48Oh, don't do this to me, please...
0:50:53 > 0:50:56The idea was to get the camera to float
0:50:56 > 0:50:59smoothly over the surface of the mound,
0:50:59 > 0:51:03using a counterweight system to pull it up the slope at a steady pace.
0:51:06 > 0:51:08How steady was that?
0:51:08 > 0:51:09Excellent.
0:51:28 > 0:51:31Huw, please don't send me back again!
0:51:35 > 0:51:37If you're listening, please...!
0:51:39 > 0:51:42One month in this muck!
0:51:42 > 0:51:46Nobody should have to live one month in...poo.
0:51:47 > 0:51:51While Gomantong was the most unpleasant cave to work in,
0:51:51 > 0:51:54the real test for the team's nerves
0:51:54 > 0:51:59was the deepest and longest of their explorations - Lechuguilla.
0:51:59 > 0:52:04Well, it seems that every cave trip involves at least one thrilling
0:52:04 > 0:52:07but buttock-clenching moment, and this is Lechuguilla's.
0:52:07 > 0:52:12It's Boulder Falls - 150-ft drop into...blackness.
0:52:14 > 0:52:17This terrifying descent into the black
0:52:17 > 0:52:20was just the beginning of a journey to reach
0:52:20 > 0:52:24Lechuguilla's most remarkable cavern, the Chandelier Ballroom.
0:52:27 > 0:52:31It was going to take the team nine hours to reach their base camp.
0:52:31 > 0:52:36And, as they descended, the passages got narrower and narrower.
0:52:36 > 0:52:40The claustrophobia of squeezing through shoulder-width crevices
0:52:40 > 0:52:44is everybody's ultimate caving nightmare.
0:52:44 > 0:52:46If you got stuck in a hole like that,
0:52:46 > 0:52:49they'd have to break your collarbone to pull you out.
0:52:49 > 0:52:53It's the only way they can bend the human body. ..Argh!
0:52:56 > 0:53:02This precipitous drop into the abyss is aptly named Freak-Out Traverse.
0:53:09 > 0:53:12Negotiating collapsed roof falls
0:53:12 > 0:53:15and climbing UP almost as much as going down,
0:53:15 > 0:53:20the team laboriously pick their way through a maze of passages.
0:53:20 > 0:53:22It was crucial that nobody fell.
0:53:22 > 0:53:25The last time somebody broke an ankle down here,
0:53:25 > 0:53:30it took a team of over 100 expert cavers 3 days to get them out.
0:53:34 > 0:53:40Finally, the team reach base camp - over a mile from the surface.
0:53:42 > 0:53:47Local experts recommend a maximum of five days below ground.
0:53:47 > 0:53:50It's only too easy to go stir crazy
0:53:50 > 0:53:53in this world without daylight or fresh air.
0:53:53 > 0:53:57But since filming underground is so time-consuming, the team had to spend
0:53:57 > 0:54:01ten days to do justice to this beautiful cave system,
0:54:01 > 0:54:04without ever coming to the surface.
0:54:07 > 0:54:10Without the normal daylight cycle to influence them,
0:54:10 > 0:54:12everyone worked round the clock,
0:54:12 > 0:54:15lugging 500 kilos of filming equipment
0:54:15 > 0:54:17through the network of narrow tunnels.
0:54:23 > 0:54:26Eventually, the team reached the ultimate goal -
0:54:26 > 0:54:28the Chandelier Ballroom.
0:54:28 > 0:54:30These 6-metre-long crystals,
0:54:30 > 0:54:35suspended from the ballroom ceiling, have taken millions of years to grow.
0:54:35 > 0:54:38And working among such precious structures
0:54:38 > 0:54:40was extremely nerve-wracking.
0:54:47 > 0:54:52This was the first time a crew had ever been allowed to carry a crane
0:54:52 > 0:54:56into the cave - an essential tool to keep the camera on the move,
0:54:56 > 0:54:59to bring these inanimate crystals to life.
0:55:05 > 0:55:08It took over three hours to set up the first shot,
0:55:08 > 0:55:12as they had to plan every single movement in advance to avoid damaging
0:55:12 > 0:55:14any of these delicate crystals.
0:55:17 > 0:55:20This place has been millions of years in the making
0:55:20 > 0:55:23and yet virtually nobody's been down here.
0:55:23 > 0:55:27This is one of the most restrictive caves in the world.
0:55:27 > 0:55:30Just to get permission to film here has taken two years.
0:55:30 > 0:55:33So it's been an amazing privilege and, frankly,
0:55:33 > 0:55:37all that slogging through, up and down rocks and squeezing ourselves
0:55:37 > 0:55:39through very tight gaps...
0:55:39 > 0:55:42I mean, just to see these amazing cave decorations -
0:55:42 > 0:55:45it's been an unbelievable highlight.
0:55:45 > 0:55:50This was to be the last time that the authorities were going to allow
0:55:50 > 0:55:54Lechuguilla to be filmed, so this footage is the only way
0:55:54 > 0:55:57most people can be transported into this magical place.
0:56:01 > 0:56:05Lechuguilla would be allowed to return to its pristine status
0:56:05 > 0:56:11as one of the most beautiful and unspoiled environments on the planet.
0:56:35 > 0:56:38Subtitles by Red Bee Media Ltd.
0:56:38 > 0:56:41E-mail subtitling@bbc.co.uk