Gunung Leuser National Park is named after Mount Leuser (3,381m), and protects a 7,927 sq km area of rainforest in north Sumatra. Designated a Unesco world heritage site along with two other national parks, together they are known as the 'Tropical Rainforest Heritage of Sumatra'. Between 1973 and 1995, the Bohorok Orangutan Rehabilitation Centre near Bukit Lawang operated as a successful rehabilitation centre, and thanks to the centre's work the jungles of the national park are now home to a stable population of orang utans, making for rewarding nature walks. Accompanied by an expert local guide, follow the faint trails through the jungle searching for orang utans, looking and listening for telltale signs of the animals. Orang utans can be difficult to spot and, as with all authentic wildlife viewing, sightings can never be guaranteed. Other species are also present, such as white and black gibbons, the endemic Thomas's leaf monkey and various bird species.