The Similan Islands are nine uninhabited granite islands in the Andaman Sea, off the west coast of Thailand. The boulders stacked on their beaches, some bigger than the boats that brought you, carry straight on under the water. The stone cooled deep underground and cracked into blocks, and water working into the joints has been rotting it from the corners inward ever since, rounding it off above the waterline and below it alike. So the reefs here are not flat coral gardens. They are tumbled fields of smooth granite dropping forty metres to white sand, with the coral growing on top of the rock, and the shapes the weathering left are the dive sites. At Elephant Head Rock, south of Island 8, three giant boulders break the surface, stacked into tunnels, arches and narrow gullies. You thread the gaps with the current pushing you through, past whitetip sharks and stingrays settled in the sediment.
The bays are gentler. Anchored in Donald Duck Bay, under the balanced block called Sail Rock, you drop into water clear enough to follow your own shadow across the pale sand, and the coral sits close enough to the surface to watch without kicking down. Late in the season the giants arrive. From February to April, warming water and plankton blooms pull manta rays and whale sharks onto the pinnacles at Koh Bon and Koh Tachai. You give up some of your thirty-metre visibility for them: mantas banking slowly over the granite, and a shape as long as a bus crossing the open blue. Neither of those two islands is one of the nine. Similan is not a Thai word but the Malay for nine, sembilan; the park took the pair in during 1998 and kept the name. Nobody sleeps out here any more; the bungalows and campsites closed in 2018. The reef is what all of that is for. Bleaching killed most of the coral here in 1998 and again in 2010, and the recovery since is coming on slowly, over granite that has been the shape of this place all along.