"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 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 percent 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 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 hundred 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 dependant on the digested remains of food that's brought here 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 rufus-bellied eagle, but they must still survive the attacks of other, more specialised, birds of prey." "Peregrine falcons and bat hawks are the jetfighters 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." "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 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 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 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 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 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 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 bigeyes 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 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 troglodytes." "There may well be less than a hundred 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 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 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 going a centimetre a day." "In this world without sunlight these bacteria extract energy from the hydrogen sulphide gas." "Bacteria like these are known as extremofile 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 metres 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 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 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 six-metre 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?"