Caves Planet Earth


Caves

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This is our planet's final frontier -

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an inner world, where only the most adventurous dare to go.

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Beneath our feet are countless miles of cave shafts and passages.

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The Cave of Swallows in Mexico.

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400 metres to the bottom -

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deep enough to engulf the Empire State Building.

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This is the biggest cave shaft in the world,

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yet these depths were first explored

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only two years before men landed on the moon.

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Today, caves remain the least explored places on Earth.

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However, human beings are seldom the first

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to reach these black, damp places.

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Here live some of the strangest and least-known animals on the planet.

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This galaxy of little lights

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is created by thousands of living creatures.

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Any animal that lives in a cave has to cope with complete blackness.

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But in New Zealand, some have turned this darkness to their advantage.

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A silken strand is lowered from the ceiling,

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alongside hundreds of others.

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Beautiful though these threads are, they have a sinister purpose.

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This is a cave glow-worm.

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To trap its prey, it goes fishing with a line of silk

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The silk comes from glands in the glow-worm's mouth

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and is loaded with droplets of mucus.

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Each glow-worm produces dozens of these threads

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Once its lines are set,

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the glow-worm hangs from a mucus hammock

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and waits, like a patient angler.

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But the glow-worm doesn't leave everything to chance.

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That ghostly blue light

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is the result of a chemical reaction

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taking place inside a special capsule in its tail.

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The light literally shines out of its backside.

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It's a lure for attracting prey.

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Insects seem irresistibly drawn towards the source

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and then get trapped by the sticky lines.

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Once stuck, there is no escape.

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Now it's just a matter of reeling in the line

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and slowly consuming the catch alive.

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By ensnaring the insects that hatch in this cave,

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these glow-worms have solved the biggest challenge

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that permanent cave-dwellers face -

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finding a regular and reliable source of food.

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One kind of rock makes this whole underground world possible -

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limestone.

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Most of the world's caves are found within it

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and it covers nearly 10% of the Earth's surface.

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Limestone is composed of minerals derived from shells and corals.

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Although this rocky escarpment in the United States

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is now hundreds of metres above sea level,

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it was actually formed underwater.

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The limestone towers of Vietnam's Ha Long Bay

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are a reminder of this link with the sea.

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Originally this whole area would have been one solid block of limestone -

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the base of a coral reef.

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In Borneo, rain has sculpted the limestone

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into extremely sharp-sided pinnacles.

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But the dissolving power of rainwater has other,

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much more dramatic, effects underground.

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Rivers that flow over limestone often seem to completely disappear.

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When the water reaches a more resistant bed of limestone,

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its course is altered.

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Once underground the water takes on a new, more erosive power.

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During its journey from the surface,

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the water absorbs carbon dioxide from the soil,

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making it mildly acidic,

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and over millions of years this acid eats away the limestone,

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creating a maze of caverns and passages that can go on for miles.

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This is the biggest underground river passage in the world.

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So big, a jumbo jet could fly through it.

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It's Deer Cave in Borneo.

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The sheer size of Deer Cave allows some animals

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to gather there in huge numbers.

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A staggering three million wrinkle-lipped bats live here.

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The bats roost high on the walls and ceilings,

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where they're well protected from the elements

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and safe from predators.

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And while they're up there, the bats produce something very important.

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This 100-metre-high mound is made entirely of bat droppings -

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guano.

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Its surface is covered by a thick carpet of cockroaches -

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hundreds of thousands of them.

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Caves are one of the few habitats on Earth

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not directly powered by sunlight.

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In the absence of plants, this food chain is based

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on a continuous supply of bat droppings.

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The cockroaches feed on the guano

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and anything that falls into it.

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The droppings also support other types of cockroaches

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which spend part of their day resting on cave walls.

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These, in turn, become food for giant cave centipedes,

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some more than 20 centimetres long.

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Bizarrely, there are crabs here too,

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sifting through the droppings for nutrients.

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All these animals spend their entire lives within the cave.

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They're totally dependent on the digested remains of food

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that's brought in from outside.

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Each evening, in just two hours,

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three million bats leave the safety of the cave

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to hunt for insects in the forest outside.

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But not all will return.

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As they leave the cave, the stream of bats form a doughnut-shaped ring.

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The wheeling bats seem to confuse a rufous-bellied eagle

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but they must still survive the attacks of other,

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more specialised, birds of prey.

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Peregrine falcons and bat hawks are the jet fighters of the bird world.

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Good hunting will end as the light fades,

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so the bat hawks bolt their catches on the wing...

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..and fly straight back for more.

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Any bat separated from the group becomes a clear and obvious target

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and is asking for trouble.

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Yet the nightly onslaught has little impact on bat numbers.

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By the morning,

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the vast majority will be back in the safety of the cave.

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Bats are not the only commuters in these Bornean caves.

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There's a day shift as well.

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Returning from hunting in the sunlight,

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these commuters rely on their loud clicks

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to find their way through the cave passages in total darkness.

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They are cave swiftlets.

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Like bats, they use echolocation to navigate.

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We need lights to see what's going on,

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but in the pitch black the swiftlets manage unerringly

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to locate their individual nesting sites

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which are only a few centimetres across.

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It's a remarkable skill and one we still do not fully understand.

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These birds are unusual for another reason.

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Their little cup-like nests are made entirely from threads of saliva.

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It takes more than 30 days to complete one.

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Their nests are very precious objects

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and not only for the birds.

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MEN CHANT RHYTHMICALLY

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For 500 years,

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people have been harvesting the nests of cave swiftlets.

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It's a very risky business.

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With virtually no safety equipment,

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and using ladders made from forest vines,

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the gatherers climb into the highest reaches of the cave,

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often more than 60 metres from the floor.

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The work may be hazardous in the extreme but the rewards are great.

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The pure white nests of cave swiftlets

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are the main ingredient of bird's nest soup

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and, gram for gram, are worth as much as silver.

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As soon as its nest is removed, a bird will immediately build another.

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So as long as this valuable harvest is properly controlled,

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the colonies will continue to flourish.

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These Bornean caves are among the biggest in the world

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and they're still getting bigger as, each year,

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rainwater eats away a little more limestone.

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But water in caves doesn't only erode, it also builds.

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This water is loaded with dissolved limestone

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and when it meets the air in the cave,

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some of that is deposited as a mineral - calcite.

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As it builds up,

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so the calcite forms decorations that hang from the ceiling - stalactites.

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Each drop leaves behind only a minuscule amount of calcite

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but over time the process can produce some spectacular results.

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If the water seeps through the ceiling quickly,

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then the calcite is deposited on the floor...

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and that creates stalagmites.

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Variations in water flow and air currents

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produce an infinite variety of forms

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but all are created by the same process -

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the slow deposition of dissolved limestone.

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And when stalactite meets stalagmite, a column is born.

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Structures like these in North America's Carlsbad Cavern

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can take many thousands of years to develop.

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But sometimes the formations in a cave stop growing altogether.

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These flooded caves in Mexico

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have remained virtually unchanged for thousands of years.

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Since the last ice age,

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they have become cut off from the outside world,

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yet their impact on life at the surface has been huge.

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Five hundred years ago,

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they supported one of the world's great civilisations -

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the Maya.

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Mexico's Yucatan peninsula has no rivers, lakes or streams,

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so the Maya relied on the cenotes -

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the flooded entrances to the water-filled caves.

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These flooded shafts are the region's only source of open fresh water.

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The cenotes are, in effect, gigantic freshwater wells.

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Away from the life-giving rays of sunshine,

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one might not expect to find plants.

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But in the darkness of the cave tunnels,

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roots of giant tropical trees have pushed their way

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through cracks in the limestone to reach the flooded caverns.

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Without this water, the Yucatan's forest could not grow so luxuriantly.

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The Maya knew that their lives depended on this water

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but it's only with the help of today's technology

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that we've come to appreciate the full significance

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and scale of these flooded passageways.

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So far, more than 350 miles

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of underwater galleries in the Yucatan have been mapped

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but still nobody yet knows

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the true extent of this subterranean water world.

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And with good reason.

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Underwater caving is notoriously dangerous.

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When the nearest exit may be hundreds of metres or more away,

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running out of air down here would be fatal.

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To avoid getting lost, divers carry with them a spool of string.

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It becomes their lifeline... literally.

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The string also doubles as a measuring tape -

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a technique that has been used, here in Mexico,

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to chart the largest underwater cave in the world -

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all 100 miles of it.

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Cave exploration often requires you

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to push yourself through narrow gaps in the rock.

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Cavers call such places squeezes.

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The tighter the squeeze,

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the greater the chance of damaging some vital life-support system.

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In these conditions, a diver could easily become disorientated

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and that could be fatal.

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The flooded caverns can play tricks on you in other ways.

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What seems like air...isn't.

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It's just another kind of water.

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This is a halocline - a meeting of fresh and salt water.

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Fresh water from the jungle

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flows over the heavier salt water from the sea.

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The salt water layer is extremely low in oxygen,

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making it a particularly difficult place for animals to live.

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Yet some have managed it, like the remipede,

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one of the most ancient of all living crustaceans.

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The Maya understood the importance of the cenotes

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but they could never have known that these flooded passageways

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were actually the beginning of subterranean rivers,

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all of which eventually flow out to the sea.

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Salt water, unlike fresh water, does not erode limestone.

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So most sea caves are created by the mechanical pounding of waves.

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The rocky outcrops of New Zealand's Poor Knights Islands

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are riddled with sea caves and, just like those in Borneo,

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they have become important shelters for many species.

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After a day feeding in the open water,

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vast shoals of demoiselle fish return to the caves,

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which they use as a refuge from predators.

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For these fish, the caves are a night-time retreat

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but they are not the only commuters in here.

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There are other fish, working to a different schedule.

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The Big Eyes are the equivalent of bats...

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night feeders that leave the cave each evening.

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And like all cave commuters, they are most vulnerable

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at the scheduled time of departure.

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A bottle-neck funnels the exiting bats into dense concentrations,

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attracting the attention of others.

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The bats can detect the snakes using echolocation,

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but the snakes are literally in the dark. They can see nothing.

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The strikes seem to be largely hit-and-miss...

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..but the snakes have a secret weapon.

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They can actually sense each bat flying past.

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Receptors in the snake's head

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pick up the heat given off by the flying bats,

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as this thermal image shows.

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To the snakes, the bats are apparently glowing

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and this gives them something to aim at.

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This is the price that these cave commuters

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must pay for their daytime sanctuary underground.

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Small wonder, then, that there are other cave dwellers that stay put.

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Many caves are like islands,

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cut off from the outside world and from other caves.

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This isolation

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has resulted in the evolution of some very strange creatures.

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They are the cave specialists.

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Troglodytes - animals that never emerge

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from the caves or see daylight.

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These troglodytes from Thailand

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are possibly the most specialised creatures on Earth,

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for they live only in cave waterfalls.

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The entire population of these cave angelfish seems to be

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restricted to just two small caves.

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It's the same story with other troglodytes.

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There may be less than 100 Texas cave salamanders in the wild.

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And the Belizean white crab is another creature that is unique

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to just one cave system.

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Living in perpetual darkness, they have all not only lost the pigment

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in their skin, but also their eyes.

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It takes thousands of generations for eyes to be lost,

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so these species must have been isolated for a very long time.

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But the blind salamander has other highly developed sensory organs.

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Receptors in the skin detect minute movements in the water

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made by its prey.

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External gills help it to breathe in water

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that is particularly low in oxygen.

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The cave angelfish feed on bacteria in the fast-flowing water,

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keeping their grip with microscopic hooks on their fins.

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Food is often in short supply and troglodytes like the crab

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have to survive on whatever washes into the cave from outside.

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A salamander might not encounter food for several months,

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so when something does come along, it can't afford to miss it.

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It is astonishing

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that these extraordinary cave dwellers manage to survive at all,

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but one cave is so inhospitable

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that one would not expect it to contain any life whatsoever.

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The water flowing out of the Villa Luz Cave in Mexico

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is actually coloured white with sulphuric acid.

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Explorers entering this dangerous cave

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must wear respirators and carry monitors.

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Poisonous gases rise to fatal levels so quickly

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that an early warning system is essential.

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Bats survive by staying close to the skylights,

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but venturing deep into the cave is very dangerous indeed.

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The source of the toxic fumes lies several miles below.

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Hydrogen sulphide gas bubbles up from oil deposits in the Earth's crust.

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It mixes with oxygen in the water and forms sulphuric acid.

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These are not the sort of conditions in which you'd expect to find fish.

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Yet these cave mollies seem to thrive, despite the acid

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and the low levels of oxygen.

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There is, in fact, more life here than anyone would think possible,

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but the biggest surprise is something altogether more bizarre.

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These strange stalactite-like formations

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are known, rather appropriately, as "snottites".

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The drops dripping from the ends

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are sulphuric acid, strong enough to burn skin.

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The snottites are, in fact, vast colonies of bacteria,

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capable of growing a centimetre a day.

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In this world without sunlight,

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these bacteria extract energy from the hydrogen sulphide gas.

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Bacteria like these are known as "extremophiles"

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because of their ability to survive in such extreme conditions.

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And these extremophiles play another important role in this cave.

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Surprisingly, they are the basis of a food chain which supports,

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amongst other creatures, the larvae of these midges.

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Villa Luz's ecosystem was certainly very remarkable,

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but cave explorers were soon to make an even more astonishing discovery.

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Beneath this arid landscape lies a subterranean wonderland.

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Without water, one might not expect to find any caves,

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but beneath these rolling desert slopes in the United States

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lies one of the longest, deepest and most surprising caves in the world.

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Its secrets remained unknown until 1986,

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when cavers dug through several metres of loose rock

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at the bottom of this pit.

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They named the cave Lechuguilla

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and since its discovery

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more than 120 miles of passageways have been mapped.

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When the first explorers descended,

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no-one guessed at the sheer size of this cave.

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But even that was not going to be the biggest surprise.

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Little did they realise that Lechuguilla would soon

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be regarded by cavers the world over as the most beautiful of all caves.

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They were about to discover some of the most exquisite formations

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ever seen underground.

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The walls were covered with the most delicate and fragile crystals.

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Many of these crystals were made of gypsum,

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a mineral that comes from limestone,

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and there was mile after mile of them.

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Water is the creator of most caves,

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but unlike all other limestone caves,

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Lechuguilla's rock had not been eaten away by running rainwater.

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Something else was responsible.

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The only water Lechuguilla has are these wonderfully still, clear pools.

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As the explorers went deeper into the cave,

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they came across whole galleries filled with unusual formations,

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like these 5-metre cones frosted with the most delicate crystals.

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It was Lechuguilla's gypsum crystals

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that made scientists question how these caverns were formed.

0:44:510:44:57

They discovered that Lechuguilla's limestone

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had actually been eaten away

0:45:000:45:03

by sulphuric acid, cutting through literally miles of limestone.

0:45:030:45:08

And when sulphuric acid dissolves limestone, it leaves behind gypsum -

0:45:250:45:30

the basis of Lechuguilla's remarkable formations.

0:45:300:45:34

And there was one set, more than a mile from the surface,

0:45:340:45:38

that almost defied belief.

0:45:380:45:40

The Chandelier Ballroom was the ultimate discovery.

0:45:570:46:01

With its 6-metre-long crystals,

0:46:010:46:04

it's surely the most bizarre cave chamber in the world.

0:46:040:46:08

And the walls had one further surprise.

0:46:490:46:52

Extremophile bacteria were found to be feeding on the rock itself.

0:46:560:47:01

The discovery of life that exists without drawing any of its energy

0:47:070:47:11

from the sun shows us once again

0:47:110:47:14

how complex and surprising the underground world can be.

0:47:140:47:19

Each year, explorers chart over 100 miles of new cave passages.

0:47:230:47:28

But with half the world's limestone still to be explored,

0:47:300:47:34

who knows how many Lechuguillas are still waiting to be discovered?

0:47:340:47:40

Of all the habitats the Planet Earth teams had to deal with, undoubtedly,

0:48:040:48:09

the one that was to provide the most unpleasant working conditions

0:48:090:48:14

was the underground world of caves.

0:48:140:48:17

Look at that!

0:48:170:48:19

Thousands of cockroaches!

0:48:190:48:22

There are a few juveniles in the moulting phase here...

0:48:220:48:26

Just everywhere you look is cockroaches.

0:48:260:48:29

Look at that.

0:48:290:48:31

Beautiful sight.

0:48:310:48:32

Gomantong Cave is home to the world's largest concentrations

0:48:370:48:41

of cockroaches. And the team are going to have to spend a month

0:48:410:48:46

working in this massive pile of guano.

0:48:460:48:48

And we're back here with the biggest mound of doo-doo

0:48:480:48:52

you've ever seen in the world.

0:48:520:48:55

And, uh...it's a bit grim.

0:48:550:48:57

We're going to get dressed up in all the stuff.

0:48:570:49:01

You get cockroaches in your neck and down your pants...

0:49:010:49:04

Oh, God!

0:49:040:49:07

Do we have to go back up?

0:49:070:49:09

The crew's goal here was to try and convey the sheer scale

0:49:090:49:13

of this mound of droppings.

0:49:130:49:16

The aim was a continuous smooth shot from the base to the very summit.

0:49:160:49:22

Now, I'm actually taping up the gusset of my paper suit here,

0:49:220:49:27

because I've spent the last few days with it splitting.

0:49:270:49:30

I'd be sitting down waist-deep in faeces, and then you can just feel

0:49:300:49:34

the insects crawling in.

0:49:340:49:37

And it's just not an area where you want insects!

0:49:370:49:41

Every inch of the ground was covered with cockroaches.

0:49:430:49:47

As soon as you put a lens or camera down, it was engulfed by bugs.

0:49:470:49:53

Death and decay was everywhere.

0:49:530:49:57

These guys are supposed to be up on the cave roof above us,

0:50:000:50:03

but I don't think this one has any idea where he is.

0:50:030:50:06

This tracking system was a bit Heath Robinson, and keeping it working

0:50:110:50:15

in this filthy, damp environment was extremely difficult.

0:50:150:50:19

Oh, no!

0:50:200:50:22

We've just come off the wheel.

0:50:220:50:25

So many things go wrong in here with equipment -

0:50:250:50:28

it's so humid and so messy

0:50:280:50:30

that everything gets gunk in it and fuses short-circuit.

0:50:300:50:35

Terrible.

0:50:350:50:36

Absolutely terrible.

0:50:360:50:39

Can't use it.

0:50:390:50:40

Oh, don't do this to me, please...

0:50:440:50:48

The idea was to get the camera to float

0:50:530:50:56

smoothly over the surface of the mound,

0:50:560:50:59

using a counterweight system to pull it up the slope at a steady pace.

0:50:590:51:03

How steady was that?

0:51:060:51:08

Excellent.

0:51:080:51:09

Huw, please don't send me back again!

0:51:280:51:31

If you're listening, please...!

0:51:350:51:37

One month in this muck!

0:51:390:51:42

Nobody should have to live one month in...poo.

0:51:420:51:46

While Gomantong was the most unpleasant cave to work in,

0:51:470:51:51

the real test for the team's nerves

0:51:510:51:54

was the deepest and longest of their explorations - Lechuguilla.

0:51:540:51:59

Well, it seems that every cave trip involves at least one thrilling

0:51:590:52:04

but buttock-clenching moment, and this is Lechuguilla's.

0:52:040:52:07

It's Boulder Falls - 150-ft drop into...blackness.

0:52:070:52:12

This terrifying descent into the black

0:52:140:52:17

was just the beginning of a journey to reach

0:52:170:52:20

Lechuguilla's most remarkable cavern, the Chandelier Ballroom.

0:52:200:52:24

It was going to take the team nine hours to reach their base camp.

0:52:270:52:31

And, as they descended, the passages got narrower and narrower.

0:52:310:52:36

The claustrophobia of squeezing through shoulder-width crevices

0:52:360:52:40

is everybody's ultimate caving nightmare.

0:52:400:52:44

If you got stuck in a hole like that,

0:52:440:52:46

they'd have to break your collarbone to pull you out.

0:52:460:52:49

It's the only way they can bend the human body. ..Argh!

0:52:490:52:53

This precipitous drop into the abyss is aptly named Freak-Out Traverse.

0:52:560:53:02

Negotiating collapsed roof falls

0:53:090:53:12

and climbing UP almost as much as going down,

0:53:120:53:15

the team laboriously pick their way through a maze of passages.

0:53:150:53:20

It was crucial that nobody fell.

0:53:200:53:22

The last time somebody broke an ankle down here,

0:53:220:53:25

it took a team of over 100 expert cavers 3 days to get them out.

0:53:250:53:30

Finally, the team reach base camp - over a mile from the surface.

0:53:340:53:40

Local experts recommend a maximum of five days below ground.

0:53:420:53:47

It's only too easy to go stir crazy

0:53:470:53:50

in this world without daylight or fresh air.

0:53:500:53:53

But since filming underground is so time-consuming, the team had to spend

0:53:530:53:57

ten days to do justice to this beautiful cave system,

0:53:570:54:01

without ever coming to the surface.

0:54:010:54:04

Without the normal daylight cycle to influence them,

0:54:070:54:10

everyone worked round the clock,

0:54:100:54:12

lugging 500 kilos of filming equipment

0:54:120:54:15

through the network of narrow tunnels.

0:54:150:54:17

Eventually, the team reached the ultimate goal -

0:54:230:54:26

the Chandelier Ballroom.

0:54:260:54:28

These 6-metre-long crystals,

0:54:280:54:30

suspended from the ballroom ceiling, have taken millions of years to grow.

0:54:300:54:35

And working among such precious structures

0:54:350:54:38

was extremely nerve-wracking.

0:54:380:54:40

This was the first time a crew had ever been allowed to carry a crane

0:54:470:54:52

into the cave - an essential tool to keep the camera on the move,

0:54:520:54:56

to bring these inanimate crystals to life.

0:54:560:54:59

It took over three hours to set up the first shot,

0:55:050:55:08

as they had to plan every single movement in advance to avoid damaging

0:55:080:55:12

any of these delicate crystals.

0:55:120:55:14

This place has been millions of years in the making

0:55:170:55:20

and yet virtually nobody's been down here.

0:55:200:55:23

This is one of the most restrictive caves in the world.

0:55:230:55:27

Just to get permission to film here has taken two years.

0:55:270:55:30

So it's been an amazing privilege and, frankly,

0:55:300:55:33

all that slogging through, up and down rocks and squeezing ourselves

0:55:330:55:37

through very tight gaps...

0:55:370:55:39

I mean, just to see these amazing cave decorations -

0:55:390:55:42

it's been an unbelievable highlight.

0:55:420:55:45

This was to be the last time that the authorities were going to allow

0:55:450:55:50

Lechuguilla to be filmed, so this footage is the only way

0:55:500:55:54

most people can be transported into this magical place.

0:55:540:55:57

Lechuguilla would be allowed to return to its pristine status

0:56:010:56:05

as one of the most beautiful and unspoiled environments on the planet.

0:56:050:56:11

Subtitles by Red Bee Media Ltd.

0:56:350:56:38

E-mail [email protected]

0:56:380:56:41

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