Bbc planet earth 04 — caves
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 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 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.