The Similan Islands are nine islands in the Andaman Sea off the west coast of Thailand, ringed by granite boulders the size of houses that carry on below the water into caves, arches and swim-throughs, and they are the best diving in the country. The granite is why. It cooled deep underground, cracked into blocks, and was rounded off by rain and sea above the waterline and below it alike, so the reefs are not flat coral gardens but tumbled boulder fields dropping forty metres to white sand, with the coral growing on the rock; at Elephant Head Rock, south of Island 8, three giant boulders break the surface and are stacked into tunnels, arches and narrow gullies that divers thread with the swell pushing through them. Above the water the islands are the same rock: white beaches between the boulders, and Sail Rock, a balanced block on the hill above Donald Duck Bay, is the classic anchorage and the picture on the postcards.
The big animals come in the spring. From February to April warming water and plankton blooms draw whale sharks and manta rays to the pinnacles at Koh Bon and Koh Tachai, and divers trade a little clarity for the season's best sightings, mantas banking slowly over the granite and the biggest fish in the sea drifting past. Snorkellers slip off the boats into the calm bays between the boulders and float over coral just below the surface with parrotfish and turtles. The name means nine, from the Malay word, and the islands are numbered one to nine running south to north; the park has since grown to eleven, and the crews still call them by number. Nobody sleeps out here any more: overnight stays were banned in 2018, the park admits a capped number of day visitors counted at the pier, and it closes entirely from May to October for the monsoon. The Similans are granite boulders that go on underwater, and the diving is what happens on the way down.