Borneo is the third largest island in the world and is located at the centre of Maritime Southeast Asia. Administratively, this island is divided between Indonesia, Malaysia and Brunei. Indonesians refer to the island as "Kalimantan." However, for people outside of Indonesia, "Kalimantan" refers to the Indonesian part of the island of Borneo. Malaysia's region of Borneo is called East Malaysia or Malaysian Borneo. The independent nation of Brunei occupies the remainder of the island. Brunei is the wealthiest nation in the island of Borneo. Borneo 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. It has an area of 743,330 km² (287,000 square miles). To the west of Borneo are the Malay Peninsula and Sumatra. To the south is Java. To the east is the island of Sulawesi (Celebes). To the northeast is the Philippines. Borneo's highest point is Mount Kinabalu in Sabah, Malaysia, with an elevation of 4,095 m (13,435 ft) above sea level. This makes it the world's third highest island.

The largest river systems are the Kapuas River, with approximately 1,143 km (710 mi) the longest river in Indonesia, the Rajang River in Sarawak with some 562.5 km (349.5 mi) the longest river in Malaysia, the Barito River about 880 km (550 mi) long and the Mahakam River about 980 km (610 mi) long.

Borneo is also known for its extensive cave systems. Clearwater Cave has one of the world's longest underwater rivers. Deer Cave, thought to be the largest cave passage in the world, is home to over three million bats and guano accumulated to over 100 metres (330 ft) high.

Ethnic and biological diversity

There are over 30 Dayak sub-ethnic groups living in Borneo, making the population of this island one of the most varied of human social groups. The native ethnic groups are Dayak Austronesians and their languages belong to the Malayo-Polynesian language family. Some sub-ethnicities are now represented by only 30-100 individuals and are threatened with extinction. Much culture, language, ethnomusic and traditional knowledge has yet to be documented by anthropologists. Ancestral knowledge of ethnobotany and ethnozoology is useful in drug discovery (for example, bintangor plant for AIDS) or as future alternative food sources (such as sago starch for lactic acid production and sago maggots as a protein source).

Mount Kinabalu, a major center of biodiversity in Borneo.Certain indigenous Dayak people (such as the Kayan, Kenyah, Punan Bah and Penan) living on the island have been struggling for decades for their right to preserve their environment from loggers and transmigrant settlers and colonists. Land reform is needed for future development in the face of rapid economic changes.

The type of rainforests found in Borneo include the high diversity mixed dipterocarp forest, the rare peat swamp forests and heath forest.

Researchers scouring swamps in the heart of Borneo island have discovered a venomous species of snake that can change its skin color. Scientists named their find the Kapuas mud snake, and speculated it might only occur in the Kapuas River drainage system.

World Wildlife Fund has stated that 361 animal and plant species have been discovered in Borneo since 1996, underscoring its unparalleled biodiversity. In the 18 month period from July 2005 until December 2006, another 52 new species were found.