Borneo, the third largest island on Earth, occupies a central position in SE Asia but geologically remains a heart of darkness. The island has an ancient core (Palaeozoic or older) to which material was accreted during the Mesozoic and early Cenozoic. These rocks are overlain and surrounded by considerable thicknesses of younger sedimentary rocks. In Neogene times the island supplied sediment to adjacent basins at a rate similar to the Himalayas