Mulu Caves

The Mulu Caves are part of one of the most extensive cave systems on Earth, located in Malaysian Borneo's Gunong Mulu National Park in Sarawak. The park was established in 1974 and became a UNESCO World Heritage Site in 2000, and the cave system was the subject of a landmark scientific expedition in the 1980s — the Mulu '80 expedition — which mapped over 295 kilometres of passages in its first 15 months and revealed cave dimensions that changed what geologists thought was geologically possible in limestone.

Deer Cave is the largest cave passage in the world by cross-section. Its entrance is 150 metres wide and 120 metres high — enough to fly a Boeing 747 through without touching the walls. The cave passage extends for over 2 kilometres and at its widest point the cavern is so large that the sound of rainfall on the entrance does not reach the interior for several seconds. The cave is named for the deer that once used its entrance as a shelter, not for any feature visible inside today.

The Sarawak Chamber, accessible via the Kelabit Highlands trek from Deer Cave, is the largest cave chamber in the world by area — roughly 600 metres long, 415 metres wide and 80 metres high. To put that in scale: five cathedral-sized Gothic naves could fit side by side inside it. The chamber was formed by a river that once flowed through it; the river now runs deep underground, and the chamber itself is silent except for the drip of water from its walls.

The nightly emergence of bats is the cave system's most dramatic biological event. At around 18:00 each evening, Klein's leaf-nosed bats (Hipposideros cervus) and wrinkle-lipped bats (Chaerephon plicatus) — an estimated population of several million — spiral out of Deer Cave's entrance in columns that can take 30 minutes or more to clear. The event has been filmed by BBC natural history crews and is considered one of the great wildlife spectacles of Southeast Asia.

The cave formations — stalactites, stalagmites, helictites and cave pearls — are growing at the rate of roughly one metre per thousand years, which means that some of the oldest formations in the accessible chambers are several hundred thousand years old. The cave environment is a slow-motion geological laboratory: the formations respond to water flow, mineral content and air movement, and the largest columns in Deer Cave represent continuous mineral deposition over geological time scales that dwarf any human timeline.

Accessing the caves is part of the adventure. The park headquarters sits in a clearing surrounded by primary Bornean rainforest, reachable by a longboat ride up the Tutoh River or by a short flight to the Mulu airfield. From the park HQ, a network of boardwalks and jungle trails leads to the cave entrances, and the experience of walking through the rainforest to reach a cave entrance — with the sound of the jungle and the humidity — is inseparable from the caves themselves.

Deer Cave holds the world's largest known cave passage by cross-section; the Sarawak Chamber is the world's largest known cave chamber by area.

The nightly bat exodus — several million bats spiralling out in columns — is one of Southeast Asia's most dramatic wildlife spectacles.

The park's cave system was only systematically explored in the 1980s and remains one of the world's most significant geological discoveries; the UNESCO inscription in 2000 reflects its global importance.