Borneo

Geography
Borneo is surrounded by the South China sea to the north and northwest, the Sulu sea to the northeast, the Celebes and the Makassar Strait to the east, the Java sea and Karimata straight to the south. To the west of Borneo are the Malay Penninsula  and Sumatra. To the south is Java. To the east is Sulawesi, and to the northeast, the Philippines.

Borneo has significant cave systems. Clearwater Cave, for example, has one of the world's longest underground rivers.

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 and Thailand. Deeper waters separating Borneo from neighboring Sulawesi prevented a land connection to that island, creating the divide between Asian and Australia-Guinea biological regions known as Wallace's Line.