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This is our planet's final frontier - | 0:00:39 | 0:00:44 | |
an inner world, where only the most adventurous dare to go. | 0:00:46 | 0:00:51 | |
Beneath our feet are countless miles of cave shafts and passages. | 0:01:43 | 0:01:48 | |
The Cave of Swallows in Mexico. | 0:02:05 | 0:02:07 | |
400 metres to the bottom - | 0:02:07 | 0:02:09 | |
deep enough to engulf the Empire State Building. | 0:02:09 | 0:02:14 | |
This is the biggest cave shaft in the world, | 0:02:21 | 0:02:24 | |
yet these depths were first explored | 0:02:24 | 0:02:26 | |
only two years before men landed on the moon. | 0:02:26 | 0:02:30 | |
Today, caves remain the least explored places on Earth. | 0:02:33 | 0:02:39 | |
However, human beings are seldom the first | 0:02:39 | 0:02:42 | |
to reach these black, damp places. | 0:02:42 | 0:02:45 | |
Here live some of the strangest and least-known animals on the planet. | 0:02:50 | 0:02:56 | |
This galaxy of little lights | 0:03:28 | 0:03:31 | |
is created by thousands of living creatures. | 0:03:31 | 0:03:34 | |
Any animal that lives in a cave has to cope with complete blackness. | 0:03:39 | 0:03:44 | |
But in New Zealand, some have turned this darkness to their advantage. | 0:03:44 | 0:03:50 | |
A silken strand is lowered from the ceiling, | 0:04:02 | 0:04:06 | |
alongside hundreds of others. | 0:04:06 | 0:04:08 | |
Beautiful though these threads are, they have a sinister purpose. | 0:04:16 | 0:04:21 | |
This is a cave glow-worm. | 0:04:22 | 0:04:26 | |
To trap its prey, it goes fishing with a line of silk | 0:04:26 | 0:04:31 | |
The silk comes from glands in the glow-worm's mouth | 0:04:37 | 0:04:42 | |
and is loaded with droplets of mucus. | 0:04:42 | 0:04:45 | |
Each glow-worm produces dozens of these threads | 0:04:59 | 0:05:04 | |
Once its lines are set, | 0:05:06 | 0:05:08 | |
the glow-worm hangs from a mucus hammock | 0:05:08 | 0:05:11 | |
and waits, like a patient angler. | 0:05:11 | 0:05:14 | |
But the glow-worm doesn't leave everything to chance. | 0:05:18 | 0:05:22 | |
That ghostly blue light | 0:05:22 | 0:05:24 | |
is the result of a chemical reaction | 0:05:24 | 0:05:27 | |
taking place inside a special capsule in its tail. | 0:05:27 | 0:05:30 | |
The light literally shines out of its backside. | 0:05:30 | 0:05:34 | |
It's a lure for attracting prey. | 0:05:34 | 0:05:38 | |
Insects seem irresistibly drawn towards the source | 0:06:04 | 0:06:08 | |
and then get trapped by the sticky lines. | 0:06:08 | 0:06:12 | |
Once stuck, there is no escape. | 0:06:21 | 0:06:24 | |
Now it's just a matter of reeling in the line | 0:06:29 | 0:06:34 | |
and slowly consuming the catch alive. | 0:06:34 | 0:06:37 | |
By ensnaring the insects that hatch in this cave, | 0:06:52 | 0:06:56 | |
these glow-worms have solved the biggest challenge | 0:06:56 | 0:06:59 | |
that permanent cave-dwellers face - | 0:06:59 | 0:07:01 | |
finding a regular and reliable source of food. | 0:07:01 | 0:07:04 | |
One kind of rock makes this whole underground world possible - | 0:07:12 | 0:07:17 | |
limestone. | 0:07:17 | 0:07:18 | |
Most of the world's caves are found within it | 0:07:18 | 0:07:21 | |
and it covers nearly 10% of the Earth's surface. | 0:07:21 | 0:07:25 | |
Limestone is composed of minerals derived from shells and corals. | 0:07:31 | 0:07:36 | |
Although this rocky escarpment in the United States | 0:07:36 | 0:07:40 | |
is now hundreds of metres above sea level, | 0:07:40 | 0:07:43 | |
it was actually formed underwater. | 0:07:43 | 0:07:45 | |
The limestone towers of Vietnam's Ha Long Bay | 0:07:52 | 0:07:56 | |
are a reminder of this link with the sea. | 0:07:56 | 0:07:59 | |
Originally this whole area would have been one solid block of limestone - | 0:07:59 | 0:08:04 | |
the base of a coral reef. | 0:08:04 | 0:08:06 | |
In Borneo, rain has sculpted the limestone | 0:08:16 | 0:08:20 | |
into extremely sharp-sided pinnacles. | 0:08:20 | 0:08:23 | |
But the dissolving power of rainwater has other, | 0:08:37 | 0:08:40 | |
much more dramatic, effects underground. | 0:08:40 | 0:08:44 | |
Rivers that flow over limestone often seem to completely disappear. | 0:08:55 | 0:08:59 | |
When the water reaches a more resistant bed of limestone, | 0:09:14 | 0:09:17 | |
its course is altered. | 0:09:17 | 0:09:19 | |
Once underground the water takes on a new, more erosive power. | 0:09:23 | 0:09:28 | |
During its journey from the surface, | 0:09:35 | 0:09:37 | |
the water absorbs carbon dioxide from the soil, | 0:09:37 | 0:09:40 | |
making it mildly acidic, | 0:09:40 | 0:09:43 | |
and over millions of years this acid eats away the limestone, | 0:09:43 | 0:09:47 | |
creating a maze of caverns and passages that can go on for miles. | 0:09:47 | 0:09:52 | |
This is the biggest underground river passage in the world. | 0:10:31 | 0:10:36 | |
So big, a jumbo jet could fly through it. | 0:10:36 | 0:10:41 | |
It's Deer Cave in Borneo. | 0:10:41 | 0:10:43 | |
The sheer size of Deer Cave allows some animals | 0:10:55 | 0:10:59 | |
to gather there in huge numbers. | 0:10:59 | 0:11:02 | |
A staggering three million wrinkle-lipped bats live here. | 0:11:09 | 0:11:14 | |
The bats roost high on the walls and ceilings, | 0:11:17 | 0:11:21 | |
where they're well protected from the elements | 0:11:21 | 0:11:24 | |
and safe from predators. | 0:11:24 | 0:11:26 | |
And while they're up there, the bats produce something very important. | 0:11:33 | 0:11:38 | |
This 100-metre-high mound is made entirely of bat droppings - | 0:11:42 | 0:11:48 | |
guano. | 0:11:48 | 0:11:49 | |
Its surface is covered by a thick carpet of cockroaches - | 0:12:11 | 0:12:16 | |
hundreds of thousands of them. | 0:12:16 | 0:12:19 | |
Caves are one of the few habitats on Earth | 0:12:24 | 0:12:28 | |
not directly powered by sunlight. | 0:12:28 | 0:12:30 | |
In the absence of plants, this food chain is based | 0:12:30 | 0:12:35 | |
on a continuous supply of bat droppings. | 0:12:35 | 0:12:38 | |
The cockroaches feed on the guano | 0:12:50 | 0:12:54 | |
and anything that falls into it. | 0:12:54 | 0:12:56 | |
The droppings also support other types of cockroaches | 0:13:23 | 0:13:27 | |
which spend part of their day resting on cave walls. | 0:13:27 | 0:13:30 | |
These, in turn, become food for giant cave centipedes, | 0:13:35 | 0:13:39 | |
some more than 20 centimetres long. | 0:13:39 | 0:13:42 | |
Bizarrely, there are crabs here too, | 0:13:45 | 0:13:48 | |
sifting through the droppings for nutrients. | 0:13:48 | 0:13:51 | |
All these animals spend their entire lives within the cave. | 0:13:58 | 0:14:02 | |
They're totally dependent on the digested remains of food | 0:14:02 | 0:14:06 | |
that's brought in from outside. | 0:14:06 | 0:14:08 | |
Each evening, in just two hours, | 0:14:35 | 0:14:37 | |
three million bats leave the safety of the cave | 0:14:37 | 0:14:41 | |
to hunt for insects in the forest outside. | 0:14:41 | 0:14:45 | |
But not all will return. | 0:14:46 | 0:14:49 | |
As they leave the cave, the stream of bats form a doughnut-shaped ring. | 0:15:10 | 0:15:16 | |
The wheeling bats seem to confuse a rufous-bellied eagle | 0:15:19 | 0:15:23 | |
but they must still survive the attacks of other, | 0:15:23 | 0:15:26 | |
more specialised, birds of prey. | 0:15:26 | 0:15:28 | |
Peregrine falcons and bat hawks are the jet fighters of the bird world. | 0:15:40 | 0:15:44 | |
Good hunting will end as the light fades, | 0:16:16 | 0:16:19 | |
so the bat hawks bolt their catches on the wing... | 0:16:19 | 0:16:22 | |
..and fly straight back for more. | 0:16:24 | 0:16:26 | |
Any bat separated from the group becomes a clear and obvious target | 0:16:28 | 0:16:33 | |
and is asking for trouble. | 0:16:33 | 0:16:35 | |
Yet the nightly onslaught has little impact on bat numbers. | 0:16:46 | 0:16:50 | |
By the morning, | 0:16:50 | 0:16:52 | |
the vast majority will be back in the safety of the cave. | 0:16:52 | 0:16:55 | |
Bats are not the only commuters in these Bornean caves. | 0:17:05 | 0:17:09 | |
There's a day shift as well. | 0:17:09 | 0:17:12 | |
Returning from hunting in the sunlight, | 0:17:18 | 0:17:21 | |
these commuters rely on their loud clicks | 0:17:21 | 0:17:23 | |
to find their way through the cave passages in total darkness. | 0:17:23 | 0:17:27 | |
They are cave swiftlets. | 0:17:33 | 0:17:35 | |
Like bats, they use echolocation to navigate. | 0:17:35 | 0:17:39 | |
We need lights to see what's going on, | 0:17:39 | 0:17:42 | |
but in the pitch black the swiftlets manage unerringly | 0:17:42 | 0:17:46 | |
to locate their individual nesting sites | 0:17:46 | 0:17:48 | |
which are only a few centimetres across. | 0:17:48 | 0:17:51 | |
It's a remarkable skill and one we still do not fully understand. | 0:17:55 | 0:18:00 | |
These birds are unusual for another reason. | 0:18:04 | 0:18:07 | |
Their little cup-like nests are made entirely from threads of saliva. | 0:18:07 | 0:18:12 | |
It takes more than 30 days to complete one. | 0:18:18 | 0:18:22 | |
Their nests are very precious objects | 0:18:22 | 0:18:25 | |
and not only for the birds. | 0:18:25 | 0:18:27 | |
MEN CHANT RHYTHMICALLY | 0:18:29 | 0:18:32 | |
For 500 years, | 0:18:45 | 0:18:47 | |
people have been harvesting the nests of cave swiftlets. | 0:18:47 | 0:18:51 | |
It's a very risky business. | 0:19:04 | 0:19:06 | |
With virtually no safety equipment, | 0:19:06 | 0:19:08 | |
and using ladders made from forest vines, | 0:19:08 | 0:19:11 | |
the gatherers climb into the highest reaches of the cave, | 0:19:11 | 0:19:15 | |
often more than 60 metres from the floor. | 0:19:15 | 0:19:18 | |
The work may be hazardous in the extreme but the rewards are great. | 0:19:27 | 0:19:32 | |
The pure white nests of cave swiftlets | 0:19:41 | 0:19:45 | |
are the main ingredient of bird's nest soup | 0:19:45 | 0:19:47 | |
and, gram for gram, are worth as much as silver. | 0:19:47 | 0:19:52 | |
As soon as its nest is removed, a bird will immediately build another. | 0:19:53 | 0:19:58 | |
So as long as this valuable harvest is properly controlled, | 0:19:58 | 0:20:03 | |
the colonies will continue to flourish. | 0:20:03 | 0:20:05 | |
These Bornean caves are among the biggest in the world | 0:20:17 | 0:20:21 | |
and they're still getting bigger as, each year, | 0:20:21 | 0:20:24 | |
rainwater eats away a little more limestone. | 0:20:24 | 0:20:27 | |
But water in caves doesn't only erode, it also builds. | 0:20:37 | 0:20:43 | |
This water is loaded with dissolved limestone | 0:20:50 | 0:20:53 | |
and when it meets the air in the cave, | 0:20:53 | 0:20:56 | |
some of that is deposited as a mineral - calcite. | 0:20:56 | 0:21:00 | |
As it builds up, | 0:21:02 | 0:21:04 | |
so the calcite forms decorations that hang from the ceiling - stalactites. | 0:21:04 | 0:21:09 | |
Each drop leaves behind only a minuscule amount of calcite | 0:21:17 | 0:21:22 | |
but over time the process can produce some spectacular results. | 0:21:22 | 0:21:26 | |
If the water seeps through the ceiling quickly, | 0:21:56 | 0:21:59 | |
then the calcite is deposited on the floor... | 0:21:59 | 0:22:02 | |
and that creates stalagmites. | 0:22:02 | 0:22:05 | |
Variations in water flow and air currents | 0:22:14 | 0:22:18 | |
produce an infinite variety of forms | 0:22:18 | 0:22:20 | |
but all are created by the same process - | 0:22:20 | 0:22:23 | |
the slow deposition of dissolved limestone. | 0:22:23 | 0:22:27 | |
And when stalactite meets stalagmite, a column is born. | 0:22:31 | 0:22:36 | |
Structures like these in North America's Carlsbad Cavern | 0:22:59 | 0:23:03 | |
can take many thousands of years to develop. | 0:23:03 | 0:23:07 | |
But sometimes the formations in a cave stop growing altogether. | 0:23:07 | 0:23:11 | |
These flooded caves in Mexico | 0:23:39 | 0:23:41 | |
have remained virtually unchanged for thousands of years. | 0:23:41 | 0:23:46 | |
Since the last ice age, | 0:23:46 | 0:23:48 | |
they have become cut off from the outside world, | 0:23:48 | 0:23:51 | |
yet their impact on life at the surface has been huge. | 0:23:51 | 0:23:55 | |
Five hundred years ago, | 0:24:00 | 0:24:02 | |
they supported one of the world's great civilisations - | 0:24:02 | 0:24:06 | |
the Maya. | 0:24:06 | 0:24:08 | |
Mexico's Yucatan peninsula has no rivers, lakes or streams, | 0:24:13 | 0:24:18 | |
so the Maya relied on the cenotes - | 0:24:18 | 0:24:20 | |
the flooded entrances to the water-filled caves. | 0:24:20 | 0:24:24 | |
These flooded shafts are the region's only source of open fresh water. | 0:24:27 | 0:24:33 | |
The cenotes are, in effect, gigantic freshwater wells. | 0:24:35 | 0:24:41 | |
Away from the life-giving rays of sunshine, | 0:25:05 | 0:25:08 | |
one might not expect to find plants. | 0:25:08 | 0:25:12 | |
But in the darkness of the cave tunnels, | 0:25:20 | 0:25:22 | |
roots of giant tropical trees have pushed their way | 0:25:22 | 0:25:26 | |
through cracks in the limestone to reach the flooded caverns. | 0:25:26 | 0:25:29 | |
Without this water, the Yucatan's forest could not grow so luxuriantly. | 0:25:32 | 0:25:37 | |
The Maya knew that their lives depended on this water | 0:25:57 | 0:26:00 | |
but it's only with the help of today's technology | 0:26:00 | 0:26:04 | |
that we've come to appreciate the full significance | 0:26:04 | 0:26:07 | |
and scale of these flooded passageways. | 0:26:07 | 0:26:10 | |
So far, more than 350 miles | 0:26:14 | 0:26:17 | |
of underwater galleries in the Yucatan have been mapped | 0:26:17 | 0:26:22 | |
but still nobody yet knows | 0:26:22 | 0:26:24 | |
the true extent of this subterranean water world. | 0:26:24 | 0:26:28 | |
And with good reason. | 0:26:28 | 0:26:30 | |
Underwater caving is notoriously dangerous. | 0:26:32 | 0:26:36 | |
When the nearest exit may be hundreds of metres or more away, | 0:26:36 | 0:26:40 | |
running out of air down here would be fatal. | 0:26:40 | 0:26:43 | |
To avoid getting lost, divers carry with them a spool of string. | 0:26:47 | 0:26:52 | |
It becomes their lifeline... literally. | 0:26:52 | 0:26:54 | |
The string also doubles as a measuring tape - | 0:27:08 | 0:27:11 | |
a technique that has been used, here in Mexico, | 0:27:11 | 0:27:14 | |
to chart the largest underwater cave in the world - | 0:27:14 | 0:27:17 | |
all 100 miles of it. | 0:27:17 | 0:27:20 | |
Cave exploration often requires you | 0:27:28 | 0:27:31 | |
to push yourself through narrow gaps in the rock. | 0:27:31 | 0:27:34 | |
Cavers call such places squeezes. | 0:27:34 | 0:27:37 | |
The tighter the squeeze, | 0:27:47 | 0:27:49 | |
the greater the chance of damaging some vital life-support system. | 0:27:49 | 0:27:53 | |
In these conditions, a diver could easily become disorientated | 0:28:18 | 0:28:24 | |
and that could be fatal. | 0:28:24 | 0:28:26 | |
The flooded caverns can play tricks on you in other ways. | 0:28:38 | 0:28:44 | |
What seems like air...isn't. | 0:28:52 | 0:28:54 | |
It's just another kind of water. | 0:28:55 | 0:28:58 | |
This is a halocline - a meeting of fresh and salt water. | 0:29:05 | 0:29:11 | |
Fresh water from the jungle | 0:29:14 | 0:29:16 | |
flows over the heavier salt water from the sea. | 0:29:16 | 0:29:21 | |
The salt water layer is extremely low in oxygen, | 0:29:21 | 0:29:24 | |
making it a particularly difficult place for animals to live. | 0:29:24 | 0:29:28 | |
Yet some have managed it, like the remipede, | 0:29:28 | 0:29:31 | |
one of the most ancient of all living crustaceans. | 0:29:31 | 0:29:35 | |
The Maya understood the importance of the cenotes | 0:29:43 | 0:29:47 | |
but they could never have known that these flooded passageways | 0:29:47 | 0:29:51 | |
were actually the beginning of subterranean rivers, | 0:29:51 | 0:29:54 | |
all of which eventually flow out to the sea. | 0:29:54 | 0:29:58 | |
Salt water, unlike fresh water, does not erode limestone. | 0:30:07 | 0:30:12 | |
So most sea caves are created by the mechanical pounding of waves. | 0:30:12 | 0:30:18 | |
The rocky outcrops of New Zealand's Poor Knights Islands | 0:30:23 | 0:30:28 | |
are riddled with sea caves and, just like those in Borneo, | 0:30:28 | 0:30:32 | |
they have become important shelters for many species. | 0:30:32 | 0:30:35 | |
After a day feeding in the open water, | 0:30:40 | 0:30:44 | |
vast shoals of demoiselle fish return to the caves, | 0:30:44 | 0:30:47 | |
which they use as a refuge from predators. | 0:30:47 | 0:30:50 | |
For these fish, the caves are a night-time retreat | 0:30:53 | 0:30:57 | |
but they are not the only commuters in here. | 0:30:57 | 0:30:59 | |
There are other fish, working to a different schedule. | 0:30:59 | 0:31:03 | |
The Big Eyes are the equivalent of bats... | 0:31:10 | 0:31:13 | |
night feeders that leave the cave each evening. | 0:31:13 | 0:31:16 | |
And like all cave commuters, they are most vulnerable | 0:31:20 | 0:31:25 | |
at the scheduled time of departure. | 0:31:25 | 0:31:27 | |
A bottle-neck funnels the exiting bats into dense concentrations, | 0:31:38 | 0:31:44 | |
attracting the attention of others. | 0:31:44 | 0:31:46 | |
The bats can detect the snakes using echolocation, | 0:32:13 | 0:32:18 | |
but the snakes are literally in the dark. They can see nothing. | 0:32:18 | 0:32:22 | |
The strikes seem to be largely hit-and-miss... | 0:32:35 | 0:32:39 | |
..but the snakes have a secret weapon. | 0:32:41 | 0:32:44 | |
They can actually sense each bat flying past. | 0:32:44 | 0:32:48 | |
Receptors in the snake's head | 0:32:48 | 0:32:50 | |
pick up the heat given off by the flying bats, | 0:32:50 | 0:32:53 | |
as this thermal image shows. | 0:32:53 | 0:32:56 | |
To the snakes, the bats are apparently glowing | 0:33:02 | 0:33:06 | |
and this gives them something to aim at. | 0:33:06 | 0:33:09 | |
This is the price that these cave commuters | 0:33:30 | 0:33:33 | |
must pay for their daytime sanctuary underground. | 0:33:33 | 0:33:37 | |
Small wonder, then, that there are other cave dwellers that stay put. | 0:33:37 | 0:33:43 | |
Many caves are like islands, | 0:33:50 | 0:33:52 | |
cut off from the outside world and from other caves. | 0:33:52 | 0:33:56 | |
This isolation | 0:34:04 | 0:34:06 | |
has resulted in the evolution of some very strange creatures. | 0:34:06 | 0:34:10 | |
They are the cave specialists. | 0:34:12 | 0:34:15 | |
Troglodytes - animals that never emerge | 0:34:15 | 0:34:19 | |
from the caves or see daylight. | 0:34:19 | 0:34:21 | |
These troglodytes from Thailand | 0:34:34 | 0:34:37 | |
are possibly the most specialised creatures on Earth, | 0:34:37 | 0:34:41 | |
for they live only in cave waterfalls. | 0:34:41 | 0:34:44 | |
The entire population of these cave angelfish seems to be | 0:34:44 | 0:34:49 | |
restricted to just two small caves. | 0:34:49 | 0:34:52 | |
It's the same story with other troglodytes. | 0:34:56 | 0:34:58 | |
There may be less than 100 Texas cave salamanders in the wild. | 0:34:58 | 0:35:03 | |
And the Belizean white crab is another creature that is unique | 0:35:19 | 0:35:25 | |
to just one cave system. | 0:35:25 | 0:35:27 | |
Living in perpetual darkness, they have all not only lost the pigment | 0:35:31 | 0:35:36 | |
in their skin, but also their eyes. | 0:35:36 | 0:35:39 | |
It takes thousands of generations for eyes to be lost, | 0:35:41 | 0:35:44 | |
so these species must have been isolated for a very long time. | 0:35:44 | 0:35:48 | |
But the blind salamander has other highly developed sensory organs. | 0:35:54 | 0:35:59 | |
Receptors in the skin detect minute movements in the water | 0:36:02 | 0:36:06 | |
made by its prey. | 0:36:06 | 0:36:08 | |
External gills help it to breathe in water | 0:36:14 | 0:36:17 | |
that is particularly low in oxygen. | 0:36:17 | 0:36:20 | |
The cave angelfish feed on bacteria in the fast-flowing water, | 0:36:31 | 0:36:36 | |
keeping their grip with microscopic hooks on their fins. | 0:36:36 | 0:36:40 | |
Food is often in short supply and troglodytes like the crab | 0:36:51 | 0:36:56 | |
have to survive on whatever washes into the cave from outside. | 0:36:56 | 0:37:00 | |
A salamander might not encounter food for several months, | 0:37:04 | 0:37:08 | |
so when something does come along, it can't afford to miss it. | 0:37:08 | 0:37:12 | |
It is astonishing | 0:37:18 | 0:37:20 | |
that these extraordinary cave dwellers manage to survive at all, | 0:37:20 | 0:37:24 | |
but one cave is so inhospitable | 0:37:24 | 0:37:28 | |
that one would not expect it to contain any life whatsoever. | 0:37:28 | 0:37:31 | |
The water flowing out of the Villa Luz Cave in Mexico | 0:37:37 | 0:37:41 | |
is actually coloured white with sulphuric acid. | 0:37:41 | 0:37:45 | |
Explorers entering this dangerous cave | 0:37:59 | 0:38:03 | |
must wear respirators and carry monitors. | 0:38:03 | 0:38:06 | |
Poisonous gases rise to fatal levels so quickly | 0:38:06 | 0:38:10 | |
that an early warning system is essential. | 0:38:10 | 0:38:13 | |
Bats survive by staying close to the skylights, | 0:38:17 | 0:38:21 | |
but venturing deep into the cave is very dangerous indeed. | 0:38:21 | 0:38:25 | |
The source of the toxic fumes lies several miles below. | 0:38:34 | 0:38:40 | |
Hydrogen sulphide gas bubbles up from oil deposits in the Earth's crust. | 0:38:40 | 0:38:46 | |
It mixes with oxygen in the water and forms sulphuric acid. | 0:38:46 | 0:38:51 | |
These are not the sort of conditions in which you'd expect to find fish. | 0:39:02 | 0:39:06 | |
Yet these cave mollies seem to thrive, despite the acid | 0:39:06 | 0:39:10 | |
and the low levels of oxygen. | 0:39:10 | 0:39:12 | |
There is, in fact, more life here than anyone would think possible, | 0:39:15 | 0:39:19 | |
but the biggest surprise is something altogether more bizarre. | 0:39:19 | 0:39:24 | |
These strange stalactite-like formations | 0:39:34 | 0:39:38 | |
are known, rather appropriately, as "snottites". | 0:39:38 | 0:39:42 | |
The drops dripping from the ends | 0:39:42 | 0:39:45 | |
are sulphuric acid, strong enough to burn skin. | 0:39:45 | 0:39:49 | |
The snottites are, in fact, vast colonies of bacteria, | 0:39:52 | 0:39:56 | |
capable of growing a centimetre a day. | 0:39:56 | 0:40:00 | |
In this world without sunlight, | 0:40:04 | 0:40:06 | |
these bacteria extract energy from the hydrogen sulphide gas. | 0:40:06 | 0:40:11 | |
Bacteria like these are known as "extremophiles" | 0:40:15 | 0:40:20 | |
because of their ability to survive in such extreme conditions. | 0:40:20 | 0:40:24 | |
And these extremophiles play another important role in this cave. | 0:40:26 | 0:40:31 | |
Surprisingly, they are the basis of a food chain which supports, | 0:40:31 | 0:40:36 | |
amongst other creatures, the larvae of these midges. | 0:40:36 | 0:40:39 | |
Villa Luz's ecosystem was certainly very remarkable, | 0:40:49 | 0:40:53 | |
but cave explorers were soon to make an even more astonishing discovery. | 0:40:53 | 0:40:58 | |
Beneath this arid landscape lies a subterranean wonderland. | 0:41:10 | 0:41:15 | |
Without water, one might not expect to find any caves, | 0:41:21 | 0:41:25 | |
but beneath these rolling desert slopes in the United States | 0:41:25 | 0:41:29 | |
lies one of the longest, deepest and most surprising caves in the world. | 0:41:29 | 0:41:34 | |
Its secrets remained unknown until 1986, | 0:41:39 | 0:41:43 | |
when cavers dug through several metres of loose rock | 0:41:43 | 0:41:47 | |
at the bottom of this pit. | 0:41:47 | 0:41:49 | |
They named the cave Lechuguilla | 0:41:52 | 0:41:55 | |
and since its discovery | 0:41:55 | 0:41:58 | |
more than 120 miles of passageways have been mapped. | 0:41:58 | 0:42:02 | |
When the first explorers descended, | 0:42:07 | 0:42:10 | |
no-one guessed at the sheer size of this cave. | 0:42:10 | 0:42:13 | |
But even that was not going to be the biggest surprise. | 0:42:13 | 0:42:17 | |
Little did they realise that Lechuguilla would soon | 0:42:17 | 0:42:20 | |
be regarded by cavers the world over as the most beautiful of all caves. | 0:42:20 | 0:42:26 | |
They were about to discover some of the most exquisite formations | 0:42:26 | 0:42:31 | |
ever seen underground. | 0:42:31 | 0:42:32 | |
The walls were covered with the most delicate and fragile crystals. | 0:42:47 | 0:42:51 | |
Many of these crystals were made of gypsum, | 0:43:06 | 0:43:10 | |
a mineral that comes from limestone, | 0:43:10 | 0:43:13 | |
and there was mile after mile of them. | 0:43:13 | 0:43:15 | |
Water is the creator of most caves, | 0:43:32 | 0:43:36 | |
but unlike all other limestone caves, | 0:43:36 | 0:43:39 | |
Lechuguilla's rock had not been eaten away by running rainwater. | 0:43:39 | 0:43:44 | |
Something else was responsible. | 0:43:45 | 0:43:47 | |
The only water Lechuguilla has are these wonderfully still, clear pools. | 0:43:55 | 0:44:01 | |
As the explorers went deeper into the cave, | 0:44:07 | 0:44:10 | |
they came across whole galleries filled with unusual formations, | 0:44:10 | 0:44:14 | |
like these 5-metre cones frosted with the most delicate crystals. | 0:44:14 | 0:44:19 | |
It was Lechuguilla's gypsum crystals | 0:44:48 | 0:44:51 | |
that made scientists question how these caverns were formed. | 0:44:51 | 0:44:57 | |
They discovered that Lechuguilla's limestone | 0:44:57 | 0:45:00 | |
had actually been eaten away | 0:45:00 | 0:45:03 | |
by sulphuric acid, cutting through literally miles of limestone. | 0:45:03 | 0:45:08 | |
And when sulphuric acid dissolves limestone, it leaves behind gypsum - | 0:45:25 | 0:45:30 | |
the basis of Lechuguilla's remarkable formations. | 0:45:30 | 0:45:34 | |
And there was one set, more than a mile from the surface, | 0:45:34 | 0:45:38 | |
that almost defied belief. | 0:45:38 | 0:45:40 | |
The Chandelier Ballroom was the ultimate discovery. | 0:45:57 | 0:46:01 | |
With its 6-metre-long crystals, | 0:46:01 | 0:46:04 | |
it's surely the most bizarre cave chamber in the world. | 0:46:04 | 0:46:08 | |
And the walls had one further surprise. | 0:46:49 | 0:46:52 | |
Extremophile bacteria were found to be feeding on the rock itself. | 0:46:56 | 0:47:01 | |
The discovery of life that exists without drawing any of its energy | 0:47:07 | 0:47:11 | |
from the sun shows us once again | 0:47:11 | 0:47:14 | |
how complex and surprising the underground world can be. | 0:47:14 | 0:47:19 | |
Each year, explorers chart over 100 miles of new cave passages. | 0:47:23 | 0:47:28 | |
But with half the world's limestone still to be explored, | 0:47:30 | 0:47:34 | |
who knows how many Lechuguillas are still waiting to be discovered? | 0:47:34 | 0:47:40 | |
Of all the habitats the Planet Earth teams had to deal with, undoubtedly, | 0:48:04 | 0:48:09 | |
the one that was to provide the most unpleasant working conditions | 0:48:09 | 0:48:14 | |
was the underground world of caves. | 0:48:14 | 0:48:17 | |
Look at that! | 0:48:17 | 0:48:19 | |
Thousands of cockroaches! | 0:48:19 | 0:48:22 | |
There are a few juveniles in the moulting phase here... | 0:48:22 | 0:48:26 | |
Just everywhere you look is cockroaches. | 0:48:26 | 0:48:29 | |
Look at that. | 0:48:29 | 0:48:31 | |
Beautiful sight. | 0:48:31 | 0:48:32 | |
Gomantong Cave is home to the world's largest concentrations | 0:48:37 | 0:48:41 | |
of cockroaches. And the team are going to have to spend a month | 0:48:41 | 0:48:46 | |
working in this massive pile of guano. | 0:48:46 | 0:48:48 | |
And we're back here with the biggest mound of doo-doo | 0:48:48 | 0:48:52 | |
you've ever seen in the world. | 0:48:52 | 0:48:55 | |
And, uh...it's a bit grim. | 0:48:55 | 0:48:57 | |
We're going to get dressed up in all the stuff. | 0:48:57 | 0:49:01 | |
You get cockroaches in your neck and down your pants... | 0:49:01 | 0:49:04 | |
Oh, God! | 0:49:04 | 0:49:07 | |
Do we have to go back up? | 0:49:07 | 0:49:09 | |
The crew's goal here was to try and convey the sheer scale | 0:49:09 | 0:49:13 | |
of this mound of droppings. | 0:49:13 | 0:49:16 | |
The aim was a continuous smooth shot from the base to the very summit. | 0:49:16 | 0:49:22 | |
Now, I'm actually taping up the gusset of my paper suit here, | 0:49:22 | 0:49:27 | |
because I've spent the last few days with it splitting. | 0:49:27 | 0:49:30 | |
I'd be sitting down waist-deep in faeces, and then you can just feel | 0:49:30 | 0:49:34 | |
the insects crawling in. | 0:49:34 | 0:49:37 | |
And it's just not an area where you want insects! | 0:49:37 | 0:49:41 | |
Every inch of the ground was covered with cockroaches. | 0:49:43 | 0:49:47 | |
As soon as you put a lens or camera down, it was engulfed by bugs. | 0:49:47 | 0:49:53 | |
Death and decay was everywhere. | 0:49:53 | 0:49:57 | |
These guys are supposed to be up on the cave roof above us, | 0:50:00 | 0:50:03 | |
but I don't think this one has any idea where he is. | 0:50:03 | 0:50:06 | |
This tracking system was a bit Heath Robinson, and keeping it working | 0:50:11 | 0:50:15 | |
in this filthy, damp environment was extremely difficult. | 0:50:15 | 0:50:19 | |
Oh, no! | 0:50:20 | 0:50:22 | |
We've just come off the wheel. | 0:50:22 | 0:50:25 | |
So many things go wrong in here with equipment - | 0:50:25 | 0:50:28 | |
it's so humid and so messy | 0:50:28 | 0:50:30 | |
that everything gets gunk in it and fuses short-circuit. | 0:50:30 | 0:50:35 | |
Terrible. | 0:50:35 | 0:50:36 | |
Absolutely terrible. | 0:50:36 | 0:50:39 | |
Can't use it. | 0:50:39 | 0:50:40 | |
Oh, don't do this to me, please... | 0:50:44 | 0:50:48 | |
The idea was to get the camera to float | 0:50:53 | 0:50:56 | |
smoothly over the surface of the mound, | 0:50:56 | 0:50:59 | |
using a counterweight system to pull it up the slope at a steady pace. | 0:50:59 | 0:51:03 | |
How steady was that? | 0:51:06 | 0:51:08 | |
Excellent. | 0:51:08 | 0:51:09 | |
Huw, please don't send me back again! | 0:51:28 | 0:51:31 | |
If you're listening, please...! | 0:51:35 | 0:51:37 | |
One month in this muck! | 0:51:39 | 0:51:42 | |
Nobody should have to live one month in...poo. | 0:51:42 | 0:51:46 | |
While Gomantong was the most unpleasant cave to work in, | 0:51:47 | 0:51:51 | |
the real test for the team's nerves | 0:51:51 | 0:51:54 | |
was the deepest and longest of their explorations - Lechuguilla. | 0:51:54 | 0:51:59 | |
Well, it seems that every cave trip involves at least one thrilling | 0:51:59 | 0:52:04 | |
but buttock-clenching moment, and this is Lechuguilla's. | 0:52:04 | 0:52:07 | |
It's Boulder Falls - 150-ft drop into...blackness. | 0:52:07 | 0:52:12 | |
This terrifying descent into the black | 0:52:14 | 0:52:17 | |
was just the beginning of a journey to reach | 0:52:17 | 0:52:20 | |
Lechuguilla's most remarkable cavern, the Chandelier Ballroom. | 0:52:20 | 0:52:24 | |
It was going to take the team nine hours to reach their base camp. | 0:52:27 | 0:52:31 | |
And, as they descended, the passages got narrower and narrower. | 0:52:31 | 0:52:36 | |
The claustrophobia of squeezing through shoulder-width crevices | 0:52:36 | 0:52:40 | |
is everybody's ultimate caving nightmare. | 0:52:40 | 0:52:44 | |
If you got stuck in a hole like that, | 0:52:44 | 0:52:46 | |
they'd have to break your collarbone to pull you out. | 0:52:46 | 0:52:49 | |
It's the only way they can bend the human body. ..Argh! | 0:52:49 | 0:52:53 | |
This precipitous drop into the abyss is aptly named Freak-Out Traverse. | 0:52:56 | 0:53:02 | |
Negotiating collapsed roof falls | 0:53:09 | 0:53:12 | |
and climbing UP almost as much as going down, | 0:53:12 | 0:53:15 | |
the team laboriously pick their way through a maze of passages. | 0:53:15 | 0:53:20 | |
It was crucial that nobody fell. | 0:53:20 | 0:53:22 | |
The last time somebody broke an ankle down here, | 0:53:22 | 0:53:25 | |
it took a team of over 100 expert cavers 3 days to get them out. | 0:53:25 | 0:53:30 | |
Finally, the team reach base camp - over a mile from the surface. | 0:53:34 | 0:53:40 | |
Local experts recommend a maximum of five days below ground. | 0:53:42 | 0:53:47 | |
It's only too easy to go stir crazy | 0:53:47 | 0:53:50 | |
in this world without daylight or fresh air. | 0:53:50 | 0:53:53 | |
But since filming underground is so time-consuming, the team had to spend | 0:53:53 | 0:53:57 | |
ten days to do justice to this beautiful cave system, | 0:53:57 | 0:54:01 | |
without ever coming to the surface. | 0:54:01 | 0:54:04 | |
Without the normal daylight cycle to influence them, | 0:54:07 | 0:54:10 | |
everyone worked round the clock, | 0:54:10 | 0:54:12 | |
lugging 500 kilos of filming equipment | 0:54:12 | 0:54:15 | |
through the network of narrow tunnels. | 0:54:15 | 0:54:17 | |
Eventually, the team reached the ultimate goal - | 0:54:23 | 0:54:26 | |
the Chandelier Ballroom. | 0:54:26 | 0:54:28 | |
These 6-metre-long crystals, | 0:54:28 | 0:54:30 | |
suspended from the ballroom ceiling, have taken millions of years to grow. | 0:54:30 | 0:54:35 | |
And working among such precious structures | 0:54:35 | 0:54:38 | |
was extremely nerve-wracking. | 0:54:38 | 0:54:40 | |
This was the first time a crew had ever been allowed to carry a crane | 0:54:47 | 0:54:52 | |
into the cave - an essential tool to keep the camera on the move, | 0:54:52 | 0:54:56 | |
to bring these inanimate crystals to life. | 0:54:56 | 0:54:59 | |
It took over three hours to set up the first shot, | 0:55:05 | 0:55:08 | |
as they had to plan every single movement in advance to avoid damaging | 0:55:08 | 0:55:12 | |
any of these delicate crystals. | 0:55:12 | 0:55:14 | |
This place has been millions of years in the making | 0:55:17 | 0:55:20 | |
and yet virtually nobody's been down here. | 0:55:20 | 0:55:23 | |
This is one of the most restrictive caves in the world. | 0:55:23 | 0:55:27 | |
Just to get permission to film here has taken two years. | 0:55:27 | 0:55:30 | |
So it's been an amazing privilege and, frankly, | 0:55:30 | 0:55:33 | |
all that slogging through, up and down rocks and squeezing ourselves | 0:55:33 | 0:55:37 | |
through very tight gaps... | 0:55:37 | 0:55:39 | |
I mean, just to see these amazing cave decorations - | 0:55:39 | 0:55:42 | |
it's been an unbelievable highlight. | 0:55:42 | 0:55:45 | |
This was to be the last time that the authorities were going to allow | 0:55:45 | 0:55:50 | |
Lechuguilla to be filmed, so this footage is the only way | 0:55:50 | 0:55:54 | |
most people can be transported into this magical place. | 0:55:54 | 0:55:57 | |
Lechuguilla would be allowed to return to its pristine status | 0:56:01 | 0:56:05 | |
as one of the most beautiful and unspoiled environments on the planet. | 0:56:05 | 0:56:11 | |
Subtitles by Red Bee Media Ltd. | 0:56:35 | 0:56:38 | |
E-mail [email protected] | 0:56:38 | 0:56:41 |