Within Mulu National Park is the world's biggest natural enclosed space - Sarawak chamber, found in Gua Nasib Bagus. It is 2,300 ft long, 1,300 ft wide and at least 230 ft high. Other notable caves are Benarat Cavern, Deer Cave, Wind Cave and Clearwater Cave, which exposes parts of a long undergraound river going through the park.
Mulu is a very inaccessible area; the only practical way of getting there is by air, mainly from Miri airport. It is possible to travel by riverboat, but it requires a chartered long boat - take around 12 hours to reach from Miri, while flight only takes 30 minutes.