The archipelago takes its name from sembilan, the Malay word for nine — the number of granite islands charted here centuries before Thailand gazetted Mu Ko Similan National Park in 1982.
Early surveyors numbered the islands rather than naming them, one through nine, and the habit held: Ko Similan itself is island eight. Ko Bon and Ko Tachai were folded into the park boundary in 1998, extending protection north across roughly 140 square kilometres of Andaman Sea. A Similan Islands early bird tour from Khao Lak crosses about 60 kilometres of that water.
The granite is the older story. These boulders are the eroded crowns of an intrusion that cooled underground some 65 million years ago, later lifted and stripped by weather until only the hardest cores remained. Wave action rounded them. The result is the western shoreline of Ko Similan and Ko Payu — smooth grey masses stacked like ballast, with the balancing formation above Donald Duck Bay the most photographed of them. Beneath the surface, the same rock forms the swim-throughs of Elephant Head Rock, known locally as Hin Pusar.
East of the granite, the geology changes and the coral takes over. Shallow hard-coral gardens shelve away from white silica sand at Ko Miang, where the park headquarters and ranger station sit. The reefs support more than 200 hard coral species and a documented fish list running past 500. Manta rays patrol the pinnacle at Ko Bon; whale sharks appear at Ko Tachai. The Moken, sea nomads of the Andaman, used these anchorages for generations before the park existed, and place names in their language survive on the charts. Island eight and Ko Miang are the two landings named on most Similan Islands early bird tour from Khao Lak tickets.
Protection has tightened since. Overnight accommodation on the islands was withdrawn in 2016, daily visitor numbers are capped, and every foreign adult pays a 500 THB conservation fee that funds ranger patrols and reef monitoring. That shift made the mainland the base. Khao Lak snorkeling day trips and Similan Islands speedboat tours now carry almost everyone who sets foot on the sand, and Similan Islands early bird tour from Khao Lak tours were built around the park's own opening hour rather than the convenience of the pier. The islands open at 08:00 and close at 16:30, seven days a week; the window is fixed, and everything else follows it.
13 hr
