The Similan National Park was established in 1982. In 1998 the park boundary was expanded to cover Koh Tachai and Koh Bon, 2 islands to the north of the park boundary. The park covers 140 km², of which 16 km² is granite islands, formed by upwellings of hot magma during the Tertiary-Cretaceous period some 65 million years ago, then later smoothed by glacial ice and wave action of the sea.