PLANET EARTH Caves

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（诱捕） 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（石灰岩）.



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（海洋贝壳，shell还有炮弹的意思，做动词则为炮击） and corals（珊瑚）, so although this rocky escarpment in the United States is now hundreds of meters above sea level it was actually formed under water.