Gunung Mulu National Park is a rainforest sitting on top of a hollow mountain. Under the trees of Malaysian Borneo, monsoon rain and underground rivers have dissolved the limestone into at least 295 kilometres of mapped passage, and cavers keep finding more. Deer Cave has one of the widest cave passages in the world, its main gallery more than 120 metres across and high enough to lose a cathedral in. The Clearwater system, surveyed at over 226 kilometres, is among the longest anywhere, and a cold clear river still runs through it. Somewhere below all of it lies the largest cave chamber on Earth. Above ground the same rock erodes the other way, upward, into the Pinnacles: a field of razor-edged limestone blades up to 45 metres tall on the flank of Gunung Api.
The park's set piece happens at dusk. From a boardwalk below Deer Cave you wait as up to three million wrinkle-lipped bats pour out of the entrance in dark twisting ribbons and head off over the forest to hunt. On a dry evening it can go on for most of an hour. By day, plank walkways thread the show caves past floodlit stalactites and clear underground streams. A 480-metre skywalk, the longest tree-based walkway anywhere, carries you 20 metres up through the crowns of the rainforest. The hardest prize is the Pinnacles: a steep, sweat-soaked haul up Gunung Api on roots, ladders and fixed ropes, usually broken over three days from a river camp. There is still no road to Mulu, only a small airstrip, which keeps the crowds thin. The bats do not fly in wet weather, so a rainy evening means nothing comes out at all.