Gunung Leuser National Park

Listen first. Before you see anything, the forest arrives as sound. A rising, looping whoop rolls through the canopy, answered by another, then another. Water drips from leaves wider than your hand. The air is warm and wet, and it smells of moss, rotting wood and crushed green stems. Underfoot, the trail is soft and dark, laced with roots. You are standing inside one of the last great rainforests of Southeast Asia.

This is Gunung Leuser National Park, in the far north of the island of Sumatra, in Indonesia. The park stretches across the mountains of Aceh and into the neighbouring province of North Sumatra. It takes its name from Mount Leuser, one of the high peaks in its rugged interior, which rises more than three thousand metres above the sea. From the lowland river valleys to those cloud-wrapped summits, the land climbs through very different forests, one above the next.

That whooping call, by the way, almost certainly belongs to siamangs. These are the largest of the gibbons, black-furred apes with a throat sac that swells like a balloon. When a pair sings together, the sac works like a drum and pushes the sound far through the trees. Families often call in the morning to mark their territory. If you hear it, look up and scan the highest branches. A dark shape swinging arm over arm is the singer.

Now for the reason most people travel here. Gunung Leuser is home to the Sumatran orangutan, a critically endangered great ape found only on this island. The name comes from Malay and means, roughly, person of the forest. Sumatran orangutans spend almost their whole lives in the trees, more than their cousins on Borneo. Here is a detail many visitors miss: in the wild, adult orangutans are mostly solitary. A mother and her young one are the closest bond you are likely to see.

The wider landscape around the park is known as the Leuser Ecosystem. Conservationists often point out something remarkable about it. It is the only place on Earth where Sumatran orangutans, tigers, rhinos and elephants still share the same forest. All four are critically endangered. You will very probably not meet a tiger or a rhino on a day walk — they are rare and shy. But their tracks, and the knowledge that they are out there, change how the forest feels.

The protection of this land has a long story. The first reserves in the area were set up during the Dutch colonial period, in the first half of the twentieth century, after pressure from local leaders and naturalists. Indonesia declared Gunung Leuser a national park in 1980. In 2004, it became part of a UNESCO World Heritage Site called the Tropical Rainforest Heritage of Sumatra, together with two other national parks on the island.

That status brought recognition, but also a warning. In 2011, UNESCO placed the site on its List of World Heritage in Danger. The main threats are illegal logging, road building, poaching and the spread of plantations, especially oil palm, along the forest edge. So the green wall in front of you is not only scenery. It is contested ground, and local rangers, researchers and communities work every day to keep it standing.

On the Aceh side, one name matters to scientists. In the Alas valley lies Ketambe, a research station where orangutans have been studied in the wild since the early 1970s. Much of what the world knows about how these apes eat, travel and raise their young comes from long, patient fieldwork in forests like these. Researchers learned, for example, that a young orangutan may stay with its mother for many years, learning which fruits ripen when and where.

The forest is just as rich at the scale of a single plant. This region of Sumatra is known for the rafflesia, a parasitic flower that has no leaves, no stem and no roots of its own. When it blooms, it opens into a heavy, rust-red disc spotted with pale marks. It smells of rotting meat, which draws the flies that pollinate it. The blooms are rare and last only a few days. If your guide shows you one, you have been lucky.

The people who live around the park are part of its story too. In this part of Aceh, the Alas and the Gayo are among the local communities. Their farms, villages and rivers sit right against the forest boundary. Many guides grew up in these valleys and learned the trails from older relatives. Ask them what the forest gives their families — water, fish, medicine, stories. Their answers often teach more than any sign.

Water shapes everything here. The Alas River runs through the valley near the town of Kutacane, brown and fast after rain, and it is popular for rafting trips. Smaller streams cut across the trails, cold and clear over grey stones. These rivers carry rain from the high forest down to farms and towns far beyond the park. Protecting the trees upstream also protects the drinking water and the rice fields downstream.

A quick word about the experience on foot. Trails can be steep, slippery and muddy, and the humidity is heavy even in the shade. Leeches are common, especially after rain. They are harmless, but they surprise people. Move slowly and quietly, and let your guide lead. Animals here hear you long before you see them. The most rewarding moments often come when the whole group stops and simply waits.

So pause here for a moment. Look at the layers around you — the dark ground, the young trees, the tall trunks with buttress roots, and the broken roof of leaves high above. Notice how the light falls in thin green beams. Listen again for the siamangs, the hornbills, the insects. When you walk back toward the forest edge, watch how the sound thins out and the air dries. You will feel the moment you leave the old forest behind.