Planet Earth (2006) s01e04 Episode Script
Caves
1
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,
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
extend of this subterranean waterworld.
And with good reason.
Underwater caving
is notoriously dangerous.
When the nearest exit may be
hundreds of meters 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 saltwater layer is extremely
low in oxygen
making it a particularly
difficult place for animals to live.
Yet some have managed it,
like the remiped, 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 Knight 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 leave
the cave each evening.
And like all cave commuters,
they are most vulnerable
at the scheduled time of departure.
A bottleneck 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
on the ground.
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 various 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 specialized
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 troglodytes.
There may well be less than a 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 their 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 troglodytes like the crab,
have to survive on whatever washes
into the cave from outside.
The 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 colored 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 and 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 snotites,
the drops dripping
from the ends are sulphuric acid,
strong enough to burn skin.
The snotites are in fact,
vast colonies of bacteria,
capable of growing a centimeter a day.
In this world without sunlight,
these bacteria extract energy
from the hydrogen sulphide gas.
Bacteria like these
are known as extremofiles
because of their ability to survive
in such extreme conditions.
And these extremofiles 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 meters of loose rock
to the bottom of this pit.
They named the cave 'Lechuguilla'
and since this 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 realize
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.
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 5-meter 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-meter
long crystals
it's surely the most bizarre
cave chamber in the world.
And the walls had
one further surprise.
Extremofile 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?
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,
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
extend of this subterranean waterworld.
And with good reason.
Underwater caving
is notoriously dangerous.
When the nearest exit may be
hundreds of meters 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 saltwater layer is extremely
low in oxygen
making it a particularly
difficult place for animals to live.
Yet some have managed it,
like the remiped, 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 Knight 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 leave
the cave each evening.
And like all cave commuters,
they are most vulnerable
at the scheduled time of departure.
A bottleneck 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
on the ground.
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 various 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 specialized
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 troglodytes.
There may well be less than a 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 their 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 troglodytes like the crab,
have to survive on whatever washes
into the cave from outside.
The 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 colored 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 and 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 snotites,
the drops dripping
from the ends are sulphuric acid,
strong enough to burn skin.
The snotites are in fact,
vast colonies of bacteria,
capable of growing a centimeter a day.
In this world without sunlight,
these bacteria extract energy
from the hydrogen sulphide gas.
Bacteria like these
are known as extremofiles
because of their ability to survive
in such extreme conditions.
And these extremofiles 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 meters of loose rock
to the bottom of this pit.
They named the cave 'Lechuguilla'
and since this 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 realize
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.
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 5-meter 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-meter
long crystals
it's surely the most bizarre
cave chamber in the world.
And the walls had
one further surprise.
Extremofile 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?