Deer Cave (Malaysia)
Credit: Mark Daffey / Getty Images

Deer Cave in Malaysian Borneo was formed when freshwater eroded the limestone that also forms Gunung Mulu National Park's famous karsts, jagged blades of rock jutting above the trees. The cave picked up its name from the animals that congregated near its opening to lick salt from the walls, but it is more commonly associated with bats, who stream out of the entrance daily. Visitors can reach the pit via a walkway that cuts through the canopy of the jungle, itself home to several types of monkeys and and more than 250 species of birds.