Mulu Caves, Sarawak

Gunung Mulu National Park in Sarawak is home to the largest cave passage on earth — the Sarawak Chamber could contain 40 Boeing 747s — and hosts a nightly spectacle when several million cave swiftlets and wrinkle-lipped bats spiral out of Deer Cave in a column visible for kilometres. The park's razor-edged Pinnacles karst formation requires two days of jungle trekking to reach and is worth every step.