After an hourlong flight across the Java Sea from Jakarta, I landed at Pangkalan Bun, near the southern coast of Borneo, to be greeted by my forest guide, Erwin. The world’s third-largest island and slightly bigger than Texas, Borneo is shared by the Indonesian province of Kalimantan, the Malaysian states of Sabah and Sarawak, and the tiny, oil-rich nation of Brunei.

Until the second half of the last century, nearly the whole island was covered by inaccessible rainforest, home to a majority of the world’s orangutans. (The only other significant population, a different subspecies, is found in Sumatra). During the 1980s and ’90s, the forests were felled at an unprecedented rate, chiefly to clear the land for highly profitable oil palm plantations. (Palm oil is used in soaps and processed foods, as well as for cooking.) Today there are thought to be approximately 100,000 orangutans left in Borneo, of which around 6,000 live in Tanjung Puting National Park, a 1,200-square-mile reserve that lies directly to the southeast of Pangkalan Bun.

Erwin summoned a taxi, and 20 minutes later we were standing on a quayside in the port of Kumai, swaddled in humidity, surveying the muddy-brown expanse of the Kumai River, which seemed to be flowing directly from the pages of a Joseph Conrad novel. A colorful assortment of local trading craft was tied up along the riverbank, while the skyline was dominated by large, enigmatic concrete structures. According to Erwin, these generated the town’s principal source of income, being nesting houses for the edible-nest swiftlets that manufacture the essential ingredient — which sells for about $1,000 a pound — of the famous Chinese soup.