Gunung Mulu Caves

Mulu protects a limestone massif riddled with some of the world's largest known cave passages, under primary Borneo rainforest, and it is reachable only by air. The Sarawak Chamber is among the biggest cave chambers ever measured, large enough that its scale is genuinely hard to convey; Deer Cave has one of the largest cave passages open to visitors, and its ceiling is home to millions of wrinkle-lipped bats. Every evening, weather permitting, they stream out in ribbons that twist across the sky for close to an hour while bat hawks pick at the edges, watched from a purpose-built observatory. Above the caves, the Pinnacles are a forest of razor-sharp limestone blades on Gunung Api, reached by one of the hardest short climbs in Southeast Asia.

What you actually see

• Deer Cave and the bat exodus - the evening flight of millions of bats from a vast cave mouth.
• Clearwater Cave - one of the world's longest cave systems, entered by longboat up the river.
• Lang Cave - the most decorated of the show caves, with dense formations.
• The Pinnacles - razor limestone blades on Gunung Api, a brutal two-day climb.

When to go

Year-round, with drier conditions from May to September. The bat exodus does not happen every evening; heavy rain keeps them inside, so allow two or three nights rather than one. Book flights and park accommodation well ahead, as capacity is small.

Honest expectations

Everything here is guided and separately charged, and the costs stack up faster than most people expect. There is no road in and flights are small and weather-dependent, so delays strand people. The Pinnacles climb is genuinely severe, with a strict turnaround time, and every year people underestimate it. Leeches are a fact of the forest walks.

Nearby

Miri is the coastal gateway; Niah Caves with their archaeological record are further along the Sarawak coast.