Interior of Deer Cave at Mulu National Park, Sarawak, Borneo, Malaysia. An explorer stands on the cave floor (lower centre). 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 which roost on the upper reaches. These bats release huge amounts of dung (guano) which collect into huge piles and provide food for a large number of species.