Emerald Cave, Ko Muk

Tham Morakot, the Emerald Cave, is a sea tunnel about eighty metres long through the limestone of Ko Muk in Trang, opening at its inner end into a collapsed doline - a circular beach of white sand enclosed by vertical cliffs on all sides with sky above and no other way in or out. Getting there means swimming the tunnel, most of it in complete darkness, with a guide leading on a rope and a torch; life jackets are provided and required. The swim is short but the darkness and the enclosed water unsettle people, and anyone uncomfortable in confined spaces should not attempt it. Tide governs access completely: at high water the tunnel roof drops too low and the cave is closed. Pirates are said to have used the hidden beach, which may or may not be true. Day boats arrive together and the small beach can hold a hundred people at once.

What you actually see

• The tunnel swim - about eighty metres, mostly in darkness, on a guide rope.
• The hidden beach - a sand floor enclosed by cliffs with sky above.
• The tide constraint - the tunnel impassable at high water.
• The crowding - day boats arriving together onto a very small beach.

When to go

November to April, and only within the tidal window - operators schedule around it and the cave closes at high water. Early trips avoid the worst of the crowding.

Honest expectations

The swim is in the dark through a confined tunnel and is genuinely unpleasant for anyone claustrophobic or a weak swimmer. Access is entirely tide-dependent. The inner beach is small and gets very crowded. Life jacket and guide are mandatory.

Nearby

Ko Kradan and Ko Ngai are close; Pak Meng and the Trang mainland are east.