Borneo is the third-largest island in the world and the largest in Asia. At the geographic centre of Maritime Southeast Asia, in relation to major Indonesian islands, it is located north of Java, west of Sulawesi, and east of Sumatra.
The island is politically divided among three countries: Malaysia and Brunei in the north, and Indonesia to the south. It is the only island in the world to be politically administered by three countries at a time. Approximately 73% of the island is Indonesian territory. In the north, the East Malaysian states of Sabah and Sarawak make up about 26% of the island. Additionally, the Malaysian federal territory of Labuan is situated on a small island just off the coast of Borneo. The sovereign state of Brunei, located on the north coast, comprises about 1% of Borneo's land area. A little more than half of the island is in the Northern Hemisphere including Brunei and the Malaysian portion, while the Indonesian portion spans both the Northern and Southern hemispheres.
Borneo is home to one of the oldest rainforests in the world.
In August 2019, Indonesian president Joko Widodo announced a plan to move the capital of Indonesia from Jakarta to a newly established location in the East Kalimantan province in Borneo.
Borneo was formed through Mesozoic accretion of microcontinental fragments, ophiolite terranes and island arc crust onto a Paleozoic continental core. At the beginning of the Cenozoic Borneo formed a promontory of Sundaland which partly separated from Asian mainland by the proto-South China Sea. The oceanic part of the proto-South China Sea was subducted during the Paleogene period and a large accretionary complex formed along the northwestern of the island of Borneo. In the early Miocene uplift of the accretionary complex occurred as a result of underthrusting of thinned continental crust in northwest. The uplift may have also resulted from shortening due to the counter-clockwise rotation of Borneo between 20 and 10 mega-annum (Ma) as a consequence of Australia-Southeast Asia collision. Large volumes of sediment were shed into basins, which scattered offshore to the west, north and east of Borneo as well into·a Neogene basin which is currently exposed in large areas of eastern and southern Sabah. In southeast Sabah, the Miocene to recent island arc terranes of the Sulu Archipelago extend onshore into Borneo with the older volcanic arc was the result of southeast dipping subduction while the younger volcanics are likely resulted from northwest dipping subduction the Celebes Sea.
Before sea levels rose at the end of the last Ice Age, Borneo was part of the mainland of Asia, forming, with Java and Sumatra, the upland regions of a peninsula that extended east from present day Indochina. The South China Sea and Gulf of Thailand now submerge the former low-lying areas of the peninsula. Deeper waters separating Borneo from neighbouring Sulawesi prevented a land connection to that island, creating the divide known as Wallace's Line between Asian and Australia-New Guinea biological regions. The island today is surrounded by the South China Sea to the north and northwest, the Sulu Sea to the northeast, the Celebes Sea and the Makassar Strait to the east, and the Java Sea and Karimata Strait to the south. To the west of Borneo are the Malay Peninsula and Sumatra. To the south and east are islands of Indonesia: Java and Sulawesi, respectively. To the northeast are the Philippine Islands. With an area of 743,330 square kilometres (287,000 sq mi), it is the third-largest island in the world, and is the largest island of Asia (the largest continent). Its highest point is Mount Kinabalu in Sabah, Malaysia, with an elevation of 4,095 m (13,435 ft).
The largest river system is the Kapuas in West Kalimantan, with a length of 1,000 km (620 mi). Other major rivers include the Mahakam in East Kalimantan (920 km (570 mi) long), the Barito, Kahayan, and Mendawai in South Kalimantan (900 km (560 mi), 600 km (370 mi), and 800 km (500 mi) long respectively), Rajang in Sarawak (565 km (351 mi) long) and Kinabatangan in Sabah (560 km (350 mi) long). Borneo has significant cave systems. In Sarawak, the Clearwater Cave has one of the world's longest underground rivers while Deer Cave is home to over three million bats, with guano accumulated to over 100 metres (330 ft) deep. The Gomantong Caves in Sabah has been dubbed as the "Cockroach Cave" due to the presence of millions of cockroaches inside the cave. The Gunung Mulu National Park in Sarawak and Sangkulirang-Mangkalihat Karst in East Kalimantan which particularly a karst areas contains thousands of smaller caves.
