Gunung Mulu National Park, Borneo
Gunung Mulu National Park in Borneo is home to one of the largest cave chambers in the world, the Sarawak Chamber. But it's famous for other reasons as well: the caves are also home to more than 12 species of bats. Researchers estimate that between 2.5 and 3.5 million wrinkle-lipped bats live in Deer Cave, another huge cave complex located within the park.
Gunung Mulu has a bat observatory, where visitors can gather each night in anticipation of the evening's exodus from the caves. The observatory is a 45-minute-to-one-hour hike from the park's entrance, so make sure to leave enough time to reach the observatory before sunset.