Gunung Mulu National Park

Listen first. The air is thick and warm, and it hums with insects. Somewhere above the canopy, a bird calls once and goes quiet. Under your feet, the boardwalk planks are damp and slightly soft. Everything smells of wet leaves, river mud and stone. This is Gunung Mulu National Park, deep in the rainforest of Sarawak, on the island of Borneo. Much of what makes it remarkable is underground.

Mulu sits in the far north of Malaysian Borneo, not far from the border with Brunei. There is no long road in from the coast. Most visitors arrive by small plane from Miri, or by a long journey upriver. That isolation has protected the place. Here, ancient rainforest meets a block of limestone pushed up from what was once a seabed. Over millions of years, rain and rivers have carved that limestone into one of the great cave landscapes on Earth.

In 2000, UNESCO listed Mulu as a World Heritage Site. The listing recognised two things together. The first is the caves, which hold some of the largest underground spaces ever measured. The second is the living forest above them, which shelters an enormous range of plants and animals. The park was created in the 1970s. Soon after, large scientific expeditions began to map it. Many of them were British, working alongside Malaysian partners, and they found far more than anyone expected.

The most famous discovery came in 1981. Explorers inside a cave called Gua Nasib Bagus — the name means Good Luck Cave — reached a chamber so large their lamps could not find the far walls. This is the Sarawak Chamber. It is counted among the largest known cave chambers in the world. Several large passenger aircraft could fit inside it. Only experienced visitors on special guided trips go there. For most people, it remains a story told in darkness.

The cave most visitors do see is Deer Cave. A walk of a few kilometres along the boardwalk brings you to its mouth. Then the scale arrives all at once. The entrance passage is one of the largest cave passages known anywhere. The ceiling rises far above you, and daylight falls into the gloom in long pale beams. The name comes from local tradition. Deer once came here to lick the salty minerals in the rock.

Here is a detail many people walk straight past. Near the entrance, look back at the rock face against the light. One edge of the cliff forms a human profile, with a strong nose and a beard. Visitors have long compared it to Abraham Lincoln, the American president. It is pure coincidence of stone and shadow. But once you see it, you cannot stop seeing it.

Deep inside Deer Cave, the passage opens to the sky again. Beyond it lies a hidden valley that guides call the Garden of Eden. The valley is enclosed by cliffs and fed by waterfalls. For a long time, it was cut off from the forest outside. You might also notice a sharp smell and a soft, dark floor. That is guano, the droppings of the colony of bats that live in the roof. An entire food chain of insects survives on it.

Those bats lead to Mulu's great evening show. On dry evenings, as the light begins to fade, the bats leave Deer Cave to hunt. They are wrinkle-lipped bats, and they number in the millions. They pour out in long ribbons that twist and curl across the sky. The shapes change constantly, perhaps to confuse the hawks that wait for them. There is a viewing area near the cave. Sit quietly, and you can hear the rush of wings.

By boat, the Melinau River leads to other caves. Clearwater Cave belongs to one of the longest cave systems in Asia. An underground river runs through it, cold and green and astonishingly clear. Nearby, Wind Cave earns its name from the cool air that moves through its passages. Inside both caves, you will see stalactites hanging from the ceiling and columns where stone has joined floor to roof. Some formations near the entrances lean towards the daylight.

That leaning is a surprise worth knowing. Near cave mouths, tiny algae and mosses grow on the forming stone. They grow towards the light, and the mineral deposits follow them. Over long ages, the formations bend, like plants turning to a window. Scientists sometimes call these photokarst. Ordinary cave formations obey gravity and hang straight down. These ones follow the sun instead.

Above ground, Mulu offers some hard and rewarding walks. The most famous is the climb to the Pinnacles on Gunung Api, a limestone mountain. Near the top, grey blades of rock rise straight out of the forest, sharp as broken glass. Reaching them takes several days, steep ladders and ropes, and good fitness. There is a gentler option too. The canopy skywalk is a chain of rope bridges strung between tall trees. It lets you walk among the birds and the upper leaves.

People have lived around Mulu for a very long time. The Berawan people and the Penan people both have deep ties to this forest. The Penan were traditionally nomadic hunters and gatherers, moving through the hills. Today, many live in settled villages near the park. On some boat trips you may stop at a small Penan market. There, people sell woven rattan baskets, bracelets and carved items. Buying directly from the makers is one simple way to support them.

Now, wherever you are standing, pause for a moment. Look at the trees around you and the limestone rising beyond them. Remember that under your feet there are rivers you cannot see. There are chambers no daylight has ever touched. As you walk on, listen for running water, a hornbill's heavy wings, or the evening chorus of insects. The forest is always busy. Most of its story happens just out of sight.