Gunung Leuser is one of the largest national parks in Southeast Asia — almost 8,000 square kilometres of rainforest, mountains and rivers spanning the border between Aceh and North Sumatra. With Kerinci Seblat and Bukit Barisan Selatan it forms the Tropical Rainforest Heritage of Sumatra, a UNESCO World Heritage Site, and it is one of the last places on earth where Sumatran orangutans, tigers, elephants and rhinos still share the same forest.
For most travellers the park means Bukit Lawang on the eastern edge, where a long-running rehabilitation programme has reintroduced semi-wild orangutans to the surrounding jungle and made them probably the most accessible great apes in the wild anywhere in the world. A two- or three-day trek out of Bukit Lawang into the forest is one of the classic Indonesian experiences. On the western side, Ketambe and Kedah are the trailheads for serious treks deeper into the park — the genuinely wild side, with fewer tourists and a much higher chance of seeing rhinos, elephants and the spectacular rafflesia flower.
The terrain ranges from lowland swamp forest to the alpine zone above 3,000 metres on Mount Leuser itself. The forest is hot, humid, leech-infested and astonishingly biodiverse. Outside the rehabilitation circuit at Bukit Lawang, all trekking inside the park requires a licensed guide, which is non-negotiable and entirely sensible — paths are often unmarked, weather changes fast, and the wildlife (especially elephants and the occasional tiger) can be genuinely dangerous.
The park is under constant pressure from illegal logging, palm oil expansion and poaching, and several of its species — including the Sumatran rhino — are now down to a handful of individuals. Visiting responsibly through licensed operators puts money directly into conservation and is the most useful thing most travellers can do.