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 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
only two years before men 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 silken 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 shells and corals.
Although this rocky escarpment in the United States
is now hundreds of metres above sea level,
it was actually formed underwater.
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 sculpted 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 a 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 absorbs 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 can 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 three million wrinkle-lipped bats live here.
The bats roost high on the walls and ceilings,
where they're well protected from the elements
and safe from predators.
And while they're up there, the bats produce something very important.
This 100-metre-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 centimetres 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 dependent on the digested remains of food
that's brought in from outside.
Each evening, in just two hours,
three 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 rufous-bellied eagle
but they must still survive the attacks of other,
more specialised, 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 are 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 centimetres 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.
Their nests are very precious objects
and not only for the birds.
MEN CHANT RHYTHMICALLY
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 metres 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 bird's 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 minuscule amount of calcite
but over time the process can produce some spectacular results.
If the water seeps through the ceiling quickly,
then the calcite is deposited on the floor...
and that creates stalagmites.
Variations in water flow and 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 have become cut off from the outside world,
yet their impact on life at the surface has been huge.
Five hundred years ago,
they supported one of the world's great civilisations -
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 freshwater 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, the Yucatan's forest could not grow so luxuriantly.
The Maya knew that their lives depended on this water
but it's only with the help of today's technology
that we've come to appreciate the full significance
and scale of these flooded passageways.
So far, more than 350 miles
of underwater galleries in the Yucatan have been mapped
but still nobody yet knows
the true extent of this subterranean water world.
And with good reason.
Underwater caving is notoriously dangerous.
When the nearest exit may be hundreds of metres or more away,
running out of air down here would be fatal.
To avoid getting lost, divers carry with them a spool of string.
It becomes their lifeline... literally.
The string also doubles as a measuring tape -
a technique that has been used, here in Mexico,
to chart the largest underwater cave in the world -
all 100 miles of it.
Cave exploration often requires you
to push yourself through narrow gaps in the rock.
Cavers call such places squeezes.
The tighter the squeeze,
the greater the chance of damaging some vital life-support system.
In these conditions, a diver could easily become disorientated
and that could be fatal.
The flooded caverns can play tricks on you in other ways.
What seems like air...isn't.
It's just another kind of water.
This is a halocline - a meeting of fresh and salt water.
Fresh water from the jungle
flows over the heavier salt water from the sea.
The salt water layer is extremely low in oxygen,
making it a particularly difficult place for animals to live.
Yet some have managed it, like the remipede,
one of the most ancient of all living crustaceans.
The Maya understood the importance of the cenotes
but they could never have known that these flooded passageways
were actually the beginning of subterranean rivers,
all of which eventually flow out to the sea.
Salt water, unlike fresh water, does not erode limestone.
So most sea caves are created by the mechanical pounding of waves.
The rocky outcrops of New Zealand's Poor Knights Islands
are riddled with sea caves and, just like those in Borneo,
they have become important shelters for 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
but they are not the only commuters in here.
There are other fish, working to a different schedule.
The Big Eyes are the equivalent of bats...
night feeders that leave the cave each evening.
And like all cave commuters, they are most vulnerable
at the scheduled time of departure.
A bottle-neck funnels the exiting bats into dense concentrations,
attracting the attention of others.
The bats can detect the snakes using echolocation,
but the snakes are literally in the dark. They 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 the heat given off by the flying bats,
as this thermal image shows.
To the snakes, the bats are apparently glowing
and this gives them something to aim at.
This is the price that these cave commuters
must pay for their daytime sanctuary underground.
Small wonder, then, that there are other cave dwellers that stay put.
Many caves are like islands,
cut off from the outside world and from other caves.
This isolation
has resulted in the evolution of some very strange creatures.
They are the cave specialists.
Troglodytes - animals that never emerge
from the caves or see daylight.
These troglodytes from Thailand
are possibly the most specialised creatures on Earth,
for they live only in cave waterfalls.
The entire population of these cave angelfish seems to be
restricted to just two small caves.
It's the same story with other troglodytes.
There may be less than 100 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
in their skin, but also their eyes.
It takes thousands of generations for eyes to be lost,
so these species must have been isolated for a very long time.
But the blind salamander has other highly developed sensory organs.
Receptors in the skin detect minute movements in the water
made by its prey.
External gills help it to breathe in water
that is particularly low in oxygen.
The cave angelfish feed on bacteria in the fast-flowing water,
keeping their grip with microscopic hooks on their fins.
Food is often in short supply and troglodytes like the crab
have to survive on whatever washes into the cave from outside.
A salamander might not encounter food for several months,
so when something does come along, it can't afford to miss it.
It is astonishing
that these extraordinary cave dwellers manage to survive at all,
but one cave is so inhospitable
that one would not expect it to contain any life whatsoever.
The water flowing out of the Villa Luz Cave in Mexico
is actually coloured white with sulphuric acid.
Explorers entering this dangerous cave
must wear respirators and carry monitors.
Poisonous gases rise to fatal levels so quickly
that an early warning system is essential.
Bats survive by staying close to the skylights,
but venturing deep into the cave is very dangerous indeed.
The source of the toxic fumes lies several miles below.
Hydrogen sulphide gas bubbles up from oil deposits in the Earth's crust.
It mixes with oxygen in the water and forms sulphuric acid.
These are not the sort of conditions in which you'd expect to find fish.
Yet these cave mollies seem to thrive, despite the acid
and the low levels of oxygen.
There is, in fact, more life here than anyone would think possible,
but the biggest surprise is something altogether more bizarre.
These strange stalactite-like formations
are known, rather appropriately, as "snottites".
The drops dripping from the ends
are 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,
these bacteria extract energy from the hydrogen sulphide gas.
Bacteria like these are known as "extremophiles"
because of their ability to survive in such extreme conditions.
And these extremophiles play another important role in this cave.
Surprisingly, they are the basis of a food chain which supports,
amongst other creatures, the larvae of these midges.
Villa Luz's ecosystem was certainly very remarkable,
but cave explorers were soon to make an even more astonishing discovery.
Beneath this arid landscape lies a subterranean wonderland.
Without water, one might not expect to find any caves,
but beneath these rolling desert slopes in the United States
lies one of the longest, deepest and most surprising caves in the world.
Its secrets remained unknown until 1986,
when cavers dug through several metres of loose rock
at the bottom of this pit.
They named the cave Lechuguilla
and since its discovery
more than 120 miles of passageways have been mapped.
When the first explorers descended,
no-one guessed at the sheer size of this cave.
But even that was not going to be the biggest surprise.
Little did they realise that Lechuguilla would soon
be regarded by cavers the world over as the most beautiful of all caves.
They were about to discover some of the most exquisite formations
ever seen underground.
The walls were covered with the most delicate and fragile crystals.
Many of these crystals were made of gypsum,
a mineral that comes from limestone,
and there was mile after mile of them.
Water is the creator of most caves,
but unlike all other limestone caves,
Lechuguilla's rock had not been eaten away by running rainwater.
Something else was responsible.
The only water Lechuguilla has are these wonderfully still, clear pools.
As the explorers went deeper into the cave,
they came across whole galleries filled with unusual formations,
like these 5-metre cones frosted with the most delicate crystals.
It was Lechuguilla's gypsum crystals
that made scientists question how these caverns were formed.
They discovered that Lechuguilla's limestone
had actually been eaten away
by sulphuric acid, cutting through literally miles of limestone.
And when sulphuric acid dissolves limestone, it leaves behind gypsum -
the basis of Lechuguilla's remarkable formations.
And there was one set, more than a mile from the surface,
that almost defied belief.
The Chandelier Ballroom was the ultimate discovery.
With its 6-metre-long crystals,
it's surely the most bizarre cave chamber in the world.
And the walls had one further surprise.
Extremophile bacteria were found to be feeding on the rock itself.
The discovery of life that exists without drawing any of its energy
from the sun shows us once again
how complex and surprising the underground world can be.
Each year, explorers chart over 100 miles of new cave passages.
But with half the world's limestone still to be explored,
who knows how many Lechuguillas are still waiting to be discovered?
Of all the habitats the Planet Earth teams had to deal with, undoubtedly,
the one that was to provide the most unpleasant working conditions
was the underground world of caves.
Look at that!
Thousands of cockroaches!
There are a few juveniles in the moulting phase here...
Just everywhere you look is cockroaches.
Look at that.
Beautiful sight.
Gomantong Cave is home to the world's largest concentrations
of cockroaches. And the team are going to have to spend a month
working in this massive pile of guano.
And we're back here with the biggest mound of doo-doo
you've ever seen in the world.
And, uh...it's a bit grim.
We're going to get dressed up in all the stuff.
You get cockroaches in your neck and down your pants...
Oh, God!
Do we have to go back up?
The crew's goal here was to try and convey the sheer scale
of this mound of droppings.
The aim was a continuous smooth shot from the base to the very summit.
Now, I'm actually taping up the gusset of my paper suit here,
because I've spent the last few days with it splitting.
I'd be sitting down waist-deep in faeces, and then you can just feel
the insects crawling in.
And it's just not an area where you want insects!
Every inch of the ground was covered with cockroaches.
As soon as you put a lens or camera down, it was engulfed by bugs.
Death and decay was everywhere.
These guys are supposed to be up on the cave roof above us,
but I don't think this one has any idea where he is.
This tracking system was a bit Heath Robinson, and keeping it working
in this filthy, damp environment was extremely difficult.
Oh, no!
We've just come off the wheel.
So many things go wrong in here with equipment -
it's so humid and so messy
that everything gets gunk in it and fuses short-circuit.
Terrible.
Absolutely terrible.
Can't use it.
Oh, don't do this to me, please...
The idea was to get the camera to float
smoothly over the surface of the mound,
using a counterweight system to pull it up the slope at a steady pace.
How steady was that?
Excellent.
Huw, please don't send me back again!
If you're listening, please...!
One month in this muck!
Nobody should have to live one month in...poo.
While Gomantong was the most unpleasant cave to work in,
the real test for the team's nerves
was the deepest and longest of their explorations - Lechuguilla.
Well, it seems that every cave trip involves at least one thrilling
but buttock-clenching moment, and this is Lechuguilla's.
It's Boulder Falls - 150-ft drop into...blackness.
This terrifying descent into the black
was just the beginning of a journey to reach
Lechuguilla's most remarkable cavern, the Chandelier Ballroom.
It was going to take the team nine hours to reach their base camp.
And, as they descended, the passages got narrower and narrower.
The claustrophobia of squeezing through shoulder-width crevices
is everybody's ultimate caving nightmare.
If you got stuck in a hole like that,
they'd have to break your collarbone to pull you out.
It's the only way they can bend the human body. ..Argh!
This precipitous drop into the abyss is aptly named Freak-Out Traverse.
Negotiating collapsed roof falls
and climbing UP almost as much as going down,
the team laboriously pick their way through a maze of passages.
It was crucial that nobody fell.
The last time somebody broke an ankle down here,
it took a team of over 100 expert cavers 3 days to get them out.
Finally, the team reach base camp - over a mile from the surface.
Local experts recommend a maximum of five days below ground.
It's only too easy to go stir crazy
in this world without daylight or fresh air.
But since filming underground is so time-consuming, the team had to spend
ten days to do justice to this beautiful cave system,
without ever coming to the surface.
Without the normal daylight cycle to influence them,
everyone worked round the clock,
lugging 500 kilos of filming equipment
through the network of narrow tunnels.
Eventually, the team reached the ultimate goal -
the Chandelier Ballroom.
These 6-metre-long crystals,
suspended from the ballroom ceiling, have taken millions of years to grow.
And working among such precious structures
was extremely nerve-wracking.
This was the first time a crew had ever been allowed to carry a crane
into the cave - an essential tool to keep the camera on the move,
to bring these inanimate crystals to life.
It took over three hours to set up the first shot,
as they had to plan every single movement in advance to avoid damaging
any of these delicate crystals.
This place has been millions of years in the making
and yet virtually nobody's been down here.
This is one of the most restrictive caves in the world.
Just to get permission to film here has taken two years.
So it's been an amazing privilege and, frankly,
all that slogging through, up and down rocks and squeezing ourselves
through very tight gaps...
I mean, just to see these amazing cave decorations -
it's been an unbelievable highlight.
This was to be the last time that the authorities were going to allow
Lechuguilla to be filmed, so this footage is the only way
most people can be transported into this magical place.
Lechuguilla would be allowed to return to its pristine status
as one of the most beautiful and unspoiled environments on the planet.
Subtitles by Red Bee Media Ltd.
E-mail subtitling@bbc.co.uk