PLANET EARTH 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（燕子洞） in Mexico, 400 meters to the bottom, deep enough to engulf the Empire State Building（帝国大厦）. 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（险恶的） 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.