Emerald Cave (Tham Morakot)

The Emerald Cave, Tham Morakot in Thai, is a sea cave in the cliffs of Koh Mook's west coast, in Trang's Hat Chao Mai National Park. You swim about 80 meters through a pitch-dark tunnel, in a life jacket and with a headlamp or a guide's flashlight, and come out into a lagoon of green water and a small beach at the bottom of a ring of cliffs open to the sky. The color that gives it its name comes from sunlight bouncing off the water, best from about 10:00 to 14:00.

The tide decides everything. The swim works best around mid-tide; high water closes the entrance and a very low tide drains the lagoon, so tours and boatmen time the visit for you. Day trips from Koh Lanta and the Trang coast arrive from about 09:00, and by the early afternoon guides lead groups of 40 through at a time, so go early. From Charlie Beach you can rent a kayak, about ฿200 an hour or ฿500 a day, and paddle there in about 30 minutes, then swim in. The park fee is ฿200 for adults and ฿100 for children, and the cave usually closes in the monsoon, roughly June to September.