PLANET EARTH Caves
The limestone towers of** Vietnam's Ha Long Bay**（越南下龙湾） 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（珊瑚礁）. In Borneo（婆罗洲，一半属于马来西亚，一半属于印尼）, 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（大型喷气式飞机） 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（栖息） high on the walls and ceilings where they're well protected from the outside elements and safe from predators.
