Planet Earth

This is our planet's final frontier.

An inner world, where only ...

the most adventurous dare to go.

Beneath our feet,

are countless miles of cave shafts and passages.

The "Cave of Swallows" in Mexico,

400 meters 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 ...

only 2 years before man landed on the moon.

Today, caves remain the least explored places on Earth.

However, human beings are seldom the first ...

to reach these black, damp places.

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

This galaxy of little lights is created by thousands of living creatures.

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

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

A silicon strand is lowered from the ceiling,

alongside hundreds of others.

Beautiful though these threads are,

they 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 in the glow worm's mouth ...

and is loaded with droplets of mucus.

Each glow worm produces dozens of these threads.

Once its lines are set,

the glow worm hangs from a mucus hammock ...

and waits, like a patient angler.

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

That ghostly blue light is the result of a chemical reaction ...

taking place inside a special capsule in its tail.

The light literally shines out of its backside.

It's a lure for attracting prey.

Insects seem irresistibly drawn towards the source ...

and then get trapped by the sticky lines.

Once 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 have solved the biggest challenge ...

that permanent cave dwellers face —

finding a regular and reliable source of food.

One kind of rock makes this whole underground world possible —

limestone.

Most of the world's caves are found within it ...

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

Limestone is composed of minerals derived from marine shells and corals,

so although this rocky escarpment in the United States ...

is now hundreds of meters above sea level,

it was actually formed under water.

The limestone towers of Vietnam's "Ha Long Bay" ...

are a reminder of this link with the sea.

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

the base of a coral reef.

In Borneo, rain has sculptured the limestone into extremely sharp-sided pinnacles.

But the dissolving power of rainwater ...

has other, much more dramatic effects underground.

Rivers that flow over limestone often seem to completely disappear.

When the water reaches the more resistant bed of limestone ...

its course is altered.

Once underground,

the water takes on a new, more erosive power.

During its journey from the surface,

the water absorbed carbon dioxide from the soil,

making it mildly acidic.

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

creating a maze of caverns and passages ...

that sometimes go on for miles.

This is the biggest underground river 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 to gather there in huge numbers.

A staggering 3 million wrinkle-lipped bats live here.

The bats roost high on the walls and ceilings ...

where they're well protected from the outside elements ...

and safe from predators.

And while they're up here,

the bats produce something very important.

This 100 meter high mound is made entirely of bat droppings —

guano.

Its surface is covered by a thick carpet of cockroaches,

hundreds of thousands of them.

Caves are one of the few habitats on Earth not 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 ...

and anything that falls into it.

The droppings also support other types of cockroaches,

which spend part of their day resting on cave walls.

These in turn become food for giant cave centipedes,

some more than 20 centimeters long.

Bizarrely, there are crabs here, too,

sifting through the droppings for nutrients.

All these animals spend their entire lives within the cave.

They're totally dependant on the digested remains of food ...

that's brought here from outside.

Each evening in just 2 hours,

3 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 ...

the stream of bats form a doughnut-shaped ring.

The wheeling bats seem to confuse a rufus-bellied eagle,

but they must still survive the attacks of other, more specialized, birds of prey.

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

Good hunting will end as the light fades,

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

and fly straight back for more.

Any bat separated from the group ...

becomes a clear and obvious target and is asking for trouble.

Yet the nightly onslaught has little impact on bat numbers —

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

Bats are not the only commuters in these Bornean caves.

There's a day shift as well.

Returning from hunting in the sunlight,

these commuters rely on their loud clicks ...

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

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 the swiftlets manage ...

unerringly to locate their individual nesting sites,

which are only a few centimeters across.

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

These birds are unusual for another reason.

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

It takes more than 30 days to complete one.

The nests are very precious objects,

and not only for the birds.

For 500 years people have been harvesting the nests of cave swiftlets.

It's a very risky business.

With virtually no safety equipment,

and using ladders made from forest vines,

the gatherers climb into the highest reaches of the cave,

often more than 60 meters from the floor.

The work may be hazardous in the extreme,

but the rewards are great.

The pure white nests of cave swiftlets ...

are the main ingredient of birds' nest soup,

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

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

So, as long as this valuable harvest is properly controlled,

the colonies will continue to flourish.

These Bornean caves are among the biggest in the world,

and they're still getting bigger ...

as each year rainwater eats away a little more limestone.

But water in caves doesn't only erode.

It also builds.

This water is loaded with dissolved limestone,

and when it meets the air in the cave,

some of that is deposited as a mineral —

calcite.

As it builds up,

so the calcite forms decorations that hang from the ceiling —

stalactites.

Each drop leaves behind only a miniscule amount of calcite,

but over time the process can produce some spectacular results.

If the water seeps though the ceiling quickly,

then the calcite is deposited on the floor of the cave ...

and that creates stalagmites.

Variations in water flow and the air currents ...

produce an infinite variety of forms, but all are created by the same process —

the slow deposition of dissolved limestone.

And when stalactite meets stalagmite,

a column is born.

Structures like these in North America's "Carlsbad Cavern" ...

can take many thousands of years to develop.

But sometimes, the formations in a cave stop growing altogether.

These flooded caves in Mexico have remained virtually unchanged for thousands of years.

Since the last Ice Age, they've become cut off from the outside world.

Yet their impact on life on the surface has been huge.

500 years ago, they supported one of the world's great civilizations:

The Maya.

Mexico's Yucatan Peninsula has no rivers, lakes or streams ...

so the Maya relied on the cenotes —

the flooded entrances to the water-filled caves.

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

The cenotes are in effect, gigantic fresh water wells.

Away from the life-giving rays of sunshine,

one might not expect to find plants.

But in the darkness of the cave tunnels,

roots of giant tropical trees,

have pushed their way through cracks in the limestone ...

to reach the flooded caverns.

Without this water,