Gunung Mulu National Park is rainforest laid over a mountain that is mostly hollow — 295 kilometres of surveyed cave passage under the trees of Malaysian Borneo — and its day turns at a cave mouth. Deer Cave is one of the widest cave passages in the world, its roof more than 120 metres over a floor deep in guano, and a plank walk takes you in. A longer guided trip wades the whole length of it and brings you out the far side into the Garden of Eden: an enclosed valley of rainforest walled in by cliffs, waterfalls dropping in from the rim, jungle nobody has cleared. A longboat up the Melinau reaches Clearwater, where a cold stream runs through one of the longest cave systems on Earth, with a jungle pool at the entrance. The largest chamber on Earth, the Sarawak Chamber, is down there too, well off the walkways. It went unmeasured for years because no lamp would throw light across it, and the cavers who first crossed its floor in 1981 never saw the far wall.
Above ground the same rock erodes the other way. The Pinnacles are a field of grey limestone blades up to 45 metres tall on the flank of Gunung Api. Getting to a ledge that looks across them takes you three days of roots, ladders and fixed ropes from a forest camp. The 480-metre skywalk is easier and nearly as high: narrow suspension bridges through the crowns, twenty metres up, hornbills at eye level. Everything comes back to dusk. You take a seat at the open-air observatory below Deer Cave and wait. The colony leaves in waves: first a trickle, then a dark unbroken column winding out over the treetops. They spiral rather than fly straight, a shape thought to help them past the bat hawks waiting at the mouth. In the wet they do not come out at all. The last of them are still going out when the cave swiftlets come back in past them, to spend the night finding their way through the dark by ear.