Rivers that flow over limestone often seem to completely disappear. When the water reaches a 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.


流过石灰岩地面的河流经常好像突然消失了。水流经过更能抗侵蚀的石灰岩河床时，会改变流向。一旦流入地下，雨水就会形成新的更大的侵蚀力。水在地表流过时候从土壤中吸收了二氧化碳，使它略带酸性，经过千百万年，这些酸性的水便把石灰岩冲蚀成洞穴迷宫，这些洞穴有的长达数公里。这是世界上最大的地下河通道，大到喷气式飞机可以通过。这是位于加里曼丹岛的鹿洞。因为洞穴很大，一些动物大量聚居在里面。