The island largely consists of dense jungle and mountains, reaching its highest point at Mt. Kinabalu (13,455 ft/4,101 m) in Sabah. Much of the terrain is virtually impassable, and large areas are unexplored. Many of the rivers are navigable to small craft, however, and provide access into the interior. The largest rivers are the Kapuas in the west and the Barito in the south. The coastal area is generally swampy and fringed with mangrove forests. Banjarmasin, Pontianak, Balikpapan, Tarakan, Kuching, Bandar Seri Begawan, and Sandakan are leading ports. The climate is tropical, i.e., hot and humid; annual rainfall averages more than 100 in. (254 cm), and there is a prolonged monsoon (generally from November to May). The fauna is roughly similar to that of Sumatra and includes the elephant, deer, orangutan, gibbon, Malay bear, and crocodile, and many varieties of snakes. Rhinoceroses, once numerous, have been extensively hunted and are now almost extinct.
The island is one of the most sparsely populated regions in the world. The two major ethnic groups are the Dyaks and the coastal Malays. Kalimantan was also a center for Chinese settlement and has a number of immigrants resettled during the second half of the 20th cent. from overcrowded areas of Indonesia, particularly Madura.