Mulu: The World'S Most Spectacular Tropical Karst

The Gunung Mulu karst of northern Sarawak hosts some of the longest caves in Southeast Asia. The majority of the cave passages in the Mulu karst are of phreatic origin and formed between sinks and springs as a series of loops crossing the bedding planes along which the passages formed. Water draining off the Mulu Sandstone also invaded the limestone down-dip, under a hydraulic gradient large enough to facilitate phreatic tubes rising up to 100 m to intersect other bedding planes. Rain falling directly on the limestone massifs has created vertical shafts and invasion vadose passages, which intersected the drained phreatic tubes, creating a three-dimensional array of cave passages. The drainage off the sandstone also migrated laterally along the contact with the limestone to produce a series of very large cavities, whose roof collapse produced a series of very large dolines or tiankeng (sky windows).