Tham Morakot (Emerald Cave)

Tham Morakot — the Emerald Cave — is the reason most day trips stop at Koh Mook: a low limestone arch on the island’s west coast that opens into an 80m pitch-black tunnel, which opens in turn onto a hidden lagoon of white sand ringed by sheer cliffs open to the sky. The cave sits inside Hat Chao Mai National Park, so entry costs 200 THB for foreign adults (100 THB for children), 40 THB for Thai adults (20 THB for children) — on top of whatever a boat operator charges for the trip out.

Getting in means swimming, not walking. Groups float or swim the tunnel together in life jackets, guided by a rope and a boat operator’s torch; the crossing takes roughly 10-15 minutes each way. The passage is genuinely dark in the middle stretch, and the ceiling drops low enough over sections that boats cancel the whole trip if the sea is rough. Access is also tide-locked — the arch floods shut at high tide, so departure times shift daily around the low-tide window, and operators visit in the morning or afternoon depending on which side of low tide falls when. Mid-tide, rather than dead-low, tends to give the clearest emerald colour in the lagoon; at the lowest tides the pool can run too shallow to glow the same way.

Watch out: non-swimmers can get through in a life jacket with a guide, but claustrophobia is the real obstacle for most people — total darkness, close rock, and the sound of other swimmers echoing off the walls for a minute or more before daylight reappears.

Insider Tip: book the first tour slot of the day. By mid-morning the lagoon fills with four-island tour groups queuing at the tunnel mouth, and the calm, uncrowded photo most people picture only happens before 9:00.

Hat Chao Mai National Park closes Ko Muk’s cave entirely from 1 June to 30 September each year, both for the monsoon swell and to let the site rest. Boats to the island run from Hat Yao pier area in Kantang district, via Khuan Tung Ku Pier (about 30 minutes to Koh Mook) or from Pak Meng Pier in Sikao (around 40 minutes); most visitors join a shared longtail or speedboat tour rather than chartering. Travellers staying overnight on the island — rather than passing through on a day tour — will find accommodation, restaurant and beach listings on the Koh Mook location page; this page covers only the cave and its access logistics.