Entrance to Deer Cave at Mulu National Park, Sarawak, Malaysia. This is believed to be one of the largest cave passages in the world with the capacity to hold five St Paul's Cathedrals. The cave is set in Gunung Mulu (Mt. Mulu), a sandstone mountain that is surrounded by limestone upheavals that are riddled with huge caves. Deer Cave derives its name from the local Penan hunters who would often trap deer there. However, the cave is also home to a huge population of bats. Every evening, millions of bats emerge from the cave to feed at night. As they do so they are preyed on by bathawks and snakes that hang from the roof of the cave mouth.