This is our planet's final frontier.
An inner world\nwhere only the most adventurous dare to go.
Beneath our feet are countless miles\nof cave shafts and passages.
The Cave of Swallows in Mexico,\n400 metres to the bottom,
deep enough to engulf the Empire State Building.
This is the biggest cave shaft in the world,
yet these depths were first explored\nonly two years before men landed on the moon.
Today, caves remain\nthe least explored places on Earth.
However, human beings are seldom\nthe first to reach these black, damp places.
Here live some of the strangest\nand least known animals on the planet.
This galaxy of little lights is created\nby thousands of living creatures.
Any animal that lives in a cave\nhas to cope with complete blackness.
But in New zealand, some have turned\nthis darkness to their advantage.
A silken strand is lowered from the ceiling\nalongside hundreds of others.
Beautiful though these threads are,\nthey have a sinister purpose.
This is a cave glow-worm.
To trap its prey, it goes fishing with a line of silk.
The silk comes from glands\nin the glow-worm's mouth
and is loaded with droplets of mucus.
Each glow-worm produces dozens\nof these threads.
0nce its lines are set,\nthe glow-worm hangs from a mucous hammock
and waits like a patient angler.
But the glow-worm\ndoesn't leave everything to chance.
That ghostly blue light\nis the result of a chemical reaction
taking place inside a special capsule in its tail.
The light literally shines out of its backside.\nIt's a lure for attracting prey.
Insects seem irresistibly drawn towards the source
and then get trapped by the sticky lines.
0nce stuck, there is no escape.
Now it's just a matter of reeling in the line
and slowly consuming the catch alive.
By ensnaring the insects that hatch in this cave,
these glow-worms\nhave solved the biggest challenge
that permanent cave-dwellers face,\nfinding a regular and reliable source of food.
0ne kind of rock makes\nthis whole underground world possible,
limestone.
Most of the world's caves are found within it\nand it covers nearly 1 0% of the Earth's surface.
Limestone is composed of minerals\nderived from marine shells and corals.
So although this rocky escarpment\nin the United States
is now hundreds of metres above sea level,\nit was actually formed underwater.
The limestone towers of Vietnam's Halong Bay\nare a reminder of this link with the sea.
0riginally this whole area\nwould have been one solid block of limestone,
the base of a coral reef.
In Borneo, rain has sculpted the limestone\ninto extremely sharp-sided pinnacles.
But the dissolving power of rainwater has other,\nmuch more dramatic, effects underground.
Rivers that flow over limestone\noften seem to completely disappear.
When the water reaches a more resistant bed\nof limestone, its course is altered.
0nce underground,\nthe water takes on a new, more erosive power.
During its journey from the surface,
the water absorbed carbon dioxide from the soil,\nmaking it mildly acidic.
And over millions of years,\nthis acid eats away the limestone,
creating a maze of caverns and passages\nthat sometimes go on for miles.
This is the biggest underground\nriver passage in the world,
so big a jumbo jet could fly through it.
It's Deer Cave in Borneo.
The sheer size of Deer Cave allows some animals\nto gather there in huge numbers.
A staggering three million\nwrinkle-lipped bats live here.
The bats roost high on the walls and ceilings,\nwhere they're well protected
from the outside elements\nand safe from predators.
And while they're up here,\nthe bats produce something very important.
This 1 00-metre-high mound\nis made entirely of bat droppings, guano.
Its surface is covered\nby a thick carpet of cockroaches.
Hundreds of thousands of them.
Caves are one of the few habitats on Earth\nnot directly powered by sunlight.
In the absence of plants, this food chain is based
on a continuous supply of bat droppings.
The cockroaches feed on the guano\nand anything that falls into it.
The droppings also support\nother types of cockroaches,
which spend part of their day\nresting on cave walls.
These in turn become food\nfor giant cave centipedes,
some more than 20 centimetres long.
Bizarrely, there are crabs here, too.\nSifting through the droppings for nutrients.
All these animals spend\ntheir entire lives within the cave.
They're totally dependent\non the digested remains of food
that's brought in from outside.
Each evening, in just two hours,\nthree million bats leave the safety of the cave
to hunt for insects in the forest outside.
But not all will return.
As they leave the cave,\nthe stream of bats form a doughnut-shaped ring.
The wheeling bats seem to confuse\na rufous-bellied eagle,
but they must still survive the attacks\nof other, more specialised birds of prey.
Peregrine falcons and bat hawks\nare the jet fighters of the bird world.
Good hunting will end as the light fades\nso the bat hawks bolt their catches on the wing
and fly straight back for more.
Any bat separated from the group\nbecomes a clear and obvious target
and is asking for trouble.
Yet the nightly onslaught\nhas little impact on bat numbers.
By the morning, the vast majority\nwill be back in the safety of the cave.
Bats are not the only commuters\nin these Bornean caves.
There's a day shift as well.
Returning from hunting in the sunlight,\nthese commuters rely on their loud clicks
to find their way through the cave passages\nin total darkness.
(CLICKING)
They're cave swiftlets.
Like bats, they use echolocation to navigate.
We need lights to see what's going on,
but in the pitch black,\nthe swiftlets manage unerringly
to locate their individual nesting sites,\nwhich are only a few centimetres across.
It's a remarkable skill\nand one we still do not fully understand.
These birds are unusual for another reason.
Their little cup-like nests\nare made entirely from threads of saliva.
It takes more than 30 days to complete one.
Their nests are very precious objects,\nand not only for the birds.
(MEN SPEAKING F0REIGN LANGUAGE)
For 500 years, people have been harvesting\nthe nests of cave swiftlets.
It's a very risky business.
With virtually no safety equipment\nand using ladders made from forest vines,
the gatherers climb\ninto the highest reaches of the cave,
often more than 60 metres from the floor.
The work may be hazardous in the extreme,\nbut the rewards are great.
The pure white nests of cave swiftlets\nare the main ingredient of bird's nest soup
and, gram for gram, are worth as much as silver.
As soon as its nest is removed,\na bird will immediately build another,
so as long as this valuable harvest\nis properly controlled,
the colonies will continue to flourish.
These Bornean caves\nare among the biggest in the world
and they're still getting bigger as, each year,\nrainwater eats away a little more limestone.
But water in caves doesn't only erode,\nit also builds.
This water is loaded with dissolved limestone,
and when it meets the air in the cave,\nsome of that is deposited as a mineral, calcite.
As it builds up, so the calcite forms decorations\nthat hang from the ceiling,
stalactites.
Each drop leaves behind\nonly a minuscule amount of calcite,
but over time,\nthe process can produce some spectacular results.
If the water seeps through the ceiling quickly,
then the calcite is deposited\non the floor of the cave,
and that creates stalagmites.
Variations in water flow and air currents\nproduce an infinite variety of forms,
but all are created by the same process,
the slow deposition of dissolved limestone.
And when stalactite meets stalagmite,\na column is born.
Structures like these\nin North America's Carlsbad Cavern
can take many thousands of years to develop.
But sometimes, the formations in a cave\nstop growing altogether.
These flooded caves in Mexico have remained\nvirtually unchanged for thousands of years.
Since the last ice age,\nthey've become cut off from the outside world,
yet their impact on life at the surface\nhas been huge.
Five hundred years ago, they supported\none of the world's great civilizations,
the Maya.
Mexico's Yucatan peninsula has no rivers,\nlakes or streams,
so the Maya relied on the ce\note\s,
the flooded entrances to the water-filled caves.
These flooded shafts\nare the region's only source of open fresh water.
The ce\note\s are, in effect,\ngigantic freshwater wells.
Away from the life-giving rays of sunshine,\none might not expect to find plants.
But in the darkness of the cave tunnels,\nroots of giant tropical trees
have pushed their way through cracks\nin the limestone to reach the flooded caverns.
Without this water,\nthe Yucatan's forest could not grow so luxuriantly.
The Maya knew that their lives\ndepended on this water,
but it's only with the help of today's technology
that we've come to appreciate the full significance\nand scale of these flooded passageways.
So far, more than 350 miles
of underwater galleries in the Yucatan\nhave been mapped.
But still, nobody yet knows the true extent\nof this subterranean water world.
And with good reason.
Underwater caving is notoriously dangerous.
When the nearest exit\nmay be hundreds of metres or more away,
running out of air down here would be fatal.
To avoid getting lost,\ndivers carry with them a spool of string.
It becomes their lifeline, literally.
The string also doubles as a measuring tape,\na technique that has been used here in Mexico
to chart the largest underwater cave\nin the world, all 1 00 miles of it.
Cave exploration often requires you\nto push yourself through narrow gaps in the rock.
Cavers call such places ''squeezes''.
The tighter the squeeze, the greater the chance\nof damaging some vital life-support system.
In these conditions,\na diver could easily become disorientated,
and that could be fatal.
The flooded caverns can play tricks on you\nin other ways.
What seems like air isn't.
It's just another kind of water.
This is a halocline,\na meeting of fresh and salt water.
Fresh water from the jungle flows\nover the heavier salt water from the sea.
The salt water layer is extremely low in oxygen,
making it a particularly difficult place\nfor animals to live.
Yet some have managed it,
like the remipede,\none of the most ancient of all living crustaceans.
The Maya understood the importance\nof the ce\note\s,
but they could never have known\nthat these flooded passageways
were actually the beginning\nof subterranean rivers,
all of which eventually flow out to the sea.
Salt water, unlike fresh water,\ndoes not erode limestone,
so most sea caves are created\nby the mechanical pounding of the waves.
The rocky outcrops\nof New zealand's Poor Knights Islands
are riddled with sea caves.
And just like those in Borneo,
they have become important shelters\nfor many species.
After a day feeding in the open water,
vast shoals of demoiselle fish return to the caves,
which they use as a refuge from predators.
For these fish, the caves are a night-time retreat,\nbut they're not the only commuters in here.
There are other fish\nworking to a different schedule.
The big-eyes are the equivalent of bats,\nnight feeders that leave the cave each evening.
And like all cave commuters,
they are most vulnerable\nat the scheduled time of departure.
A bottle-neck funnels these exiting bats\ninto dense concentrations,
attracting the attention of others.
The bats can detect the snakes using echolocation,
but the snakes are literally in the dark,\nthey can see nothing.
The strikes seem to be largely hit and miss.
But the snakes have a secret weapon.
They can actually sense each bat flying past.
Receptors in the snake's head pick up\nthe heat given off by the flying bats,
as this thermal image shows.
To the snakes, the bats are apparently glowing\nand this gives them something to aim at.
This is the price\nthat these cave commuters must pay
for their daytime sanctuary underground.
Small wonder, then,\nthat there are other cave dwellers that stay put.
Many caves are like islands,
cut off from the outside world\nand from other caves.
This isolation has resulted in the evolution\nof some very strange creatures.
They are the cave specialists, troglobites,
animals that never emerge from the caves\nor see daylight.
These troglobites from Thailand are possibly\nthe most specialised creatures on Earth,
for they live only in cave waterfalls.
The entire population of these cave angel fish\nseems to be restricted to just two small caves.
It's the same story with other troglobites.
There may well be less\nthan 1 00 Texas cave salamanders in the wild.
And the Belizean white crab is another creature
that is unique to just one cave system.
Living in perpetual darkness,
they have all not only lost the pigment\nin their skin but also their eyes.
It takes thousands of generations\nfor eyes to be lost
so these species must have been isolated\nfor a very long time.
But the blind salamander\nhas other highly-developed sensory organs.
Receptors in the skin detect minute movements\nin the water made by its prey.
External gills help it to breathe in water\nthat is particularly low in oxygen.
The cave angel fish feed on bacteria\nin the fast-flowing water,
keeping their grip\nwith microscopic hooks on their fins.
Food is often in short supply
and troglobites, like the crab, have to survive\non whatever washes into the cave from outside.
A salamander might not encounter food\nfor several months,
so when something does come along,\nit can't afford to miss it.
It's astonishing\nthat these extraordinary cave dwellers
manage to survive at all.
But one cave is so inhospitable
that one would not expect it\nto contain any life whatsoever.
The water flowing out\nof the Villa Luz cave in Mexico
is actually coloured white with sulphuric acid.
Explorers entering this dangerous cave\nmust wear respirators and carry monitors.
Poisonous gases rise to fatal levels so quickly\nthat an early warning system is essential.
Bats survive by staying close to the skylights,
but venturing deep into the cave\nis very dangerous indeed.
The source of these toxic fumes\nlies several miles below.
Hydrogen sulphide gas bubbles up\nfrom oil deposits in the Earth's crust.
It mixes with oxygen in the water\nand forms sulphuric acid.
These are not the sort of conditions\nin which you would expect to find fish.
Yet these cave mollies seem to thrive,\ndespite the acid and the low levels of oxygen.
There is, in fact, more life here\nthan anyone would think possible,
but the biggest surprise\nis something altogether more bizarre.
These strange stalactite-like formations
are known, rather appropriately, as snottites.
The drops dripping from the ends\nare sulphuric acid,
strong enough to burn skin.
The snottites are, in fact, vast colonies of bacteria
capable of growing a centimetre a day.
In this world without sunlight,\nthese bacteria extract energy
from the hydrogen sulphide gas.
Bacteria like these are known as extremophiles
because of their ability to survive\nin such extreme conditions.
And these extremophiles play\nanother important role in this cave.
Surprisingly, they are the basis of a food chain
which supports, amongst other creatures,\nthe larvae of these midges.
Villa Luz's ecosystem\nwas certainly very remarkable,
but cave explorers were soon to make\nan even more astonishing discovery.
Beneath this arid landscape\nlies a subterranean wonderland.
Without water,\none might not expect to find any caves,
but beneath these rolling desert slopes\nin the United States
lies one of the longest, deepest\nand most surprising caves in the world.
Its secrets remained unknown until 1 986,
when cavers dug through several metres\nof loose rock at the bottom of this pit.
They named the cave Lechuguilla\nand since its discovery,
more than 1 20 miles of passageways\nhave been mapped.
When the first explorers descended,\nno one guessed at the sheer size of this cave,
but even that\nwas not going to be the biggest surprise.
Little did they realise\nthat Lechuguilla would soon be regarded
by cavers the world over\nas the most beautiful of all caves.
They were about to discover\nsome of the most exquisite formations
ever seen underground.
The walls were covered\nwith the most delicate and fragile crystals.
Many of these crystals were made of gypsum,\na mineral that comes from limestone,
and there was mile after mile of them.
Water is the creator of most caves,\nbut unlike all other limestone caves,
Lechuguilla's rock had not been eaten away\nby running rainwater.
Something else was responsible.
The only water Lechuguilla has\nare these wonderfully still, clear pools.
As the explorers went deeper into the cave,
they came across whole galleries\nfilled with the most unusual formations,
like these five-metre cones\nfrosted with the most delicate crystals.
It was Lechuguilla's gypsum crystals\nthat made scientists question
how these caverns were formed.
They discovered that Lechuguilla's limestone\nhad actually been eaten away
by sulphuric acid\ncutting through literally miles of limestone.
And when sulphuric acid dissolves limestone,\nit leaves behind gypsum,
the basis of Lechuguilla's remarkable formations.
And there was one set,\nmore than a mile from the surface,
that almost defied belief.
The Chandelier Ballroom\nwas the ultimate discovery.
With its six-metre-long crystals, it's surely\nthe most bizarre cave chamber in the world.
And the walls had one further surprise.
Extremophile bacteria\nwere found to be feeding on the rock itself.
The discovery of life that exists\nwithout drawing any of its energy from the sun
shows us once again how complex and surprising\nthe underground world can be.
Each year, explorers chart over a hundred miles\nof new cave passages.
But with half the world's limestone\nstill to be explored,
who knows how many Lechuguillas\nare still waiting to be discovered?