All language subtitles for Planet Earth (2006) - S01E04 - Caves (1080p BluRay x265 Silence)

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 mucous 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 metres above sea level,
it was actually formed underwater.

The limestone towers of Vietnam's Halong 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 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 three 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-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'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 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.

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 of the cave,

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've 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 civilizations,

the Maya.

Mexico's Yucatán 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 Yucatán'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 Yucatán
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 the 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're 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 these 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, troglobites,

animals that never emerge from the caves
or see daylight.

These troglobites from Thailand are possibly
the most specialised creatures on Earth,

for they live only in cave waterfalls.

The entire population of these cave angel fish
seems to be restricted to just two small caves.

It's the same story with other troglobites.

There may well 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 angel fish feed on bacteria
in the fast-flowing water,

keeping their grip
with microscopic hooks on their fins.

Food is often in short supply

and troglobites, 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's 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 these 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 would 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 the most unusual formations,

like these five-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 six-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 a hundred 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?