Tanjung Puting National Park

Listen first. The engine of the wooden boat drops to a low knock, and the river opens around you. The water is the colour of strong tea, dark and almost black in the shade. Insects hum in the trees. Somewhere in the green wall on either bank, a branch cracks... then goes still. The air is warm and heavy, and it smells of wet leaves and mud. This is Tanjung Puting, in the south of Borneo, in the Indonesian province of Central Kalimantan.

Most visitors arrive the way you probably did, on a klotok. A klotok is a traditional wooden riverboat, named for the knocking sound of its engine. Journeys usually begin at the port town of Kumai, near Pangkalan Bun. From there the boat crosses a wide bay and turns into the Sekonyer River. For the next few days, the boat is your home. You eat on the deck, you sleep on the deck, and you watch the forest pass at walking pace.

Notice how the landscape changes as you travel upstream. Near the river mouth, the banks are lined with nipa palms. These are low palms with long fronds that grow straight out of brackish water, where salt water meets fresh. Further in, the palms give way to tall trees with their roots in soft ground. Much of this park is peat swamp forest. Peat is a thick layer of dead plant matter that never fully rots, because the ground stays waterlogged.

That peat explains the colour of the water. As rain seeps through the forest floor, it picks up tannins from decaying leaves. Tannins are the same natural compounds that colour a cup of tea. The result is a blackwater river — clear in a glass, but dark as coffee in the channel. On a calm morning, the surface acts like a mirror. The trees appear twice, once above the water and once below it.

Now the reason most people come here. Tanjung Puting is one of the most important places in the world to see wild and rehabilitated orangutans. The name orangutan comes from Malay and is usually translated as person of the forest. Bornean orangutans are found only on this island. They are the largest tree-dwelling mammals on Earth. Adult males can develop wide cheek pads, called flanges, which make their faces look almost like round dishes.

The story of orangutan research here began in 1971. That year, a young Canadian scientist named Biruté Galdikas arrived to study them in the wild. She was one of three women encouraged by the anthropologist Louis Leakey to study great apes. The other two were Jane Goodall, who worked with chimpanzees, and Dian Fossey, who worked with gorillas. Galdikas set up a simple research station deep in the forest. She named it Camp Leakey.

That camp still stands at the end of the river journey, and it remains a working research site. Over the decades, the work grew beyond observation. Orangutans orphaned by logging, forest fires, or the illegal pet trade were brought here. Staff cared for them and taught them the skills of forest life. Many were released back into the wild. Some of the orangutans you see today are descendants of those animals.

Along the river there are several feeding stations, including Tanjung Harapan, Pondok Tanggui, and Camp Leakey itself. Rangers bring bananas and other food to a wooden platform at set times. The feeding supports animals that may not find enough food in the forest, especially in lean seasons. You will walk a short trail and wait quietly. Then the leaves begin to shake. An orangutan swings in, slowly and deliberately, often with a baby clinging to its side.

Here is something many visitors do not expect. Orangutans are mostly solitary. Unlike chimpanzees, they do not live in large, noisy groups. A mother and her young form the closest bond, and young orangutans stay with their mothers for many years — longer than almost any other animal except humans. So when several orangutans appear at one platform, it is a rare social moment. Watch how carefully they keep their distance from one another.

The orangutans are not the only residents. In the late afternoon, look along the riverbanks for proboscis monkeys. They are found only on Borneo. The males have long, drooping noses and round bellies, and they gather in trees near the water to sleep. Listen too for gibbons. Their calls rise and fall across the canopy in the early morning. Hornbills may pass overhead, and their wingbeats make a heavy, whooshing sound.

The river holds its own secrets. Crocodiles live in these waters, including a slender-snouted species often called the false gharial. You may never see one, but they are the reason nobody swims here. Kingfishers flash blue and orange along the edges. And if your boat moors for the night in the right place, you may see a second surprise. Certain trees along the bank fill with fireflies, blinking together in the dark like tiny strings of light.

This forest is under pressure, and it is honest to say so. Across Borneo, large areas have been cleared for oil palm plantations and logging. Peat fires have burned during dry years, sending smoke across the region. In some stretches of the Sekonyer, the water turns cloudy and brown, a change that has been linked to mining and land clearing upstream. Protected land like this park matters, because it keeps a large piece of the original forest standing.

So take a moment now, wherever you are on the river. Feel the slow movement of the boat beneath you. Look at the dark water, the tangle of roots, the canopy closing overhead. Somewhere in those trees, an orangutan is building a nest of bent branches for the night, as it does every evening. When the engine starts again and the forest slides past, keep watching the treetops. A shaking branch may be the last farewell from the people of the forest.