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% 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 sculpted the limestone into extremely sharp-sided pinnacles. But the dissolving power of rainwater has other, much more dramatic effects underground.


有一种岩石使这种洞穴的形成成为可能，这就是石灰岩。世界上绝大多数岩洞都是由石灰岩形成的。地球表边近10%覆盖着石灰岩。石灰岩是由海洋贝壳类和珊瑚骨骼中的矿物质组成的，尽管位于美国的这座悬崖高出海平面数百米，但实际上它是在水下形成的。位于越南下龙湾的这些石灰岩山可以让我们看到它与海洋的联系。最初这一地区本是一个完整的石灰岩板块，一座珊瑚礁的礁底。在加里曼丹岛，雨水把石灰岩雕刻成了这种边缘异常尖利的石柱。但是，在地下雨水的冲蚀作用与此不同，并且更有戏剧性。