Emerald Cave (Tham Morakot), Koh Mook

Tham Morakot, the Emerald Cave, is the best-known sight in the Trang islands and the reason most day boats come to Koh Mook. An opening at the foot of the island's west cliffs leads into a tunnel about eighty metres long, which is swum rather than walked - part of it in complete darkness.

The tunnel opens into a collapsed cavern: a small beach of white sand enclosed by sheer walls hundreds of feet high, open to the sky and hung with vegetation. The name comes from the colour the water turns at the entrance when the light is right.

Trips go with a guide and life jackets, usually on a rope line, and are timed to the tide - the tunnel is impassable at high water and the cavern beach shrinks or disappears. It is not for weak swimmers or anyone uneasy in the dark or in confined spaces, and it can be crowded when several boats arrive together.