Mulu Caves, Malaysia

The limestone mountains of the Gunung Mulu National Park and the adjacent Gunung Buda National Park, in Sarawak on Borneo, contain over 400 km of cave passages mapped by a series of British and American expeditions. Many of these caves, both the active river passages and the abandoned high-levels, are very large and they include the world's largest cave passage and the world's largest cave chamber. Sediment dating has demonstrated that the caves’ major trunk passages evolved as a sequence over more than two million years in response to falling base levels as land surfaces were lowering adjacent to the limestone mountains.