Siberut is the largest and northernmost of the Mentawai Islands, lying 150 kilometres west of Sumatra in the Indian Ocean. A part of Indonesia
, the island is the most important home for the Mentawai people. The western half of the island was set aside as the Siberut National Park
in 1993. Much of the island is covered with rainforest, but is subject to commercial logging.
Smaller islands adjacent to Siberut include Karamajet and Masokut which lie in the Bungalaut strait at the south of the island. The island is known for its range of primates, including the Kloss Gibbon (Hylobates klossii), pig-tailed langur (Simias concolor), Mentawai Langur (Presbytis potenziani) and Mentawai Macaque (Macaca pagensis). Siberut was affected by the 2004 Indian Ocean earthquake tsunami, but without any known loss of human life. One report stated that the island may have been raised up to two metres by the earthquake.
Siberut Island has a hot and humid tropical rainforest climate, with an annual rainfall of 4,000 mm. Temperatures range from 22 to 31 °C, and humidity averages 81-85%. The east coast has many islets, bays and coral reefs, and is covered with mangrove forest extending up to 2 km wide before giving way to nipah forest. The west coast features mainly Barringtonia forests and is difficult to get to because of rough seas and steep cliffs. The hilly interior elevates to 384 metres, with many streams eventually forming rivers in the sago-grove lowlands and swamp forests. There are also areas of dipterocarp primary forest.
Some anthropologists believe that several thousand years ago, the Bataks of North Sumatra may have been the first to settle here. However, there are now substantial differences in culture and language among the inhabitants. The Swiss anthropologist, Reimar Schefold spent years among one of the Siberut groups, the Sakuddei.