Emerald Cave (Tham Morakot)

A sea cave on the west side of Koh Mook where you swim about 80 m through a dark tunnel to a small beach inside a ring of cliffs; the entrance opens only at low tide. Trips from Lanta combine it with snorkelling at Koh Chueak and lunch on Koh Ngai or Koh Kradan. On a training week: keep it for a rest day. The boat days from Lanta run from morning to late afternoon, with several swims, and the swim through the cave is in the dark, so you need to be comfortable in water. Salt water stings fresh cuts on the shins; a rash vest and reef-safe sunscreen help. From the camps: the trips pick up from stays on Koh Lanta, where the Lanta camps on our list are, and the boat ride to Koh Mook is about an hour.