Emerald Cave, Ko Muk

Ko Muk is one of the main islands of the Trang sea; Trang has 46 islands, 12 in Kantang, 13 in Palian and 21 in Sikao. Only the eastern cape facing the mainland is inhabited, with a small fishing village and a few resorts, while the west coast where the cave lies is forested and uninhabited. The name appears as both "Ko Muk" and "Ko Mukd".

Ko Muk lies in Hat Chao Mai National Park, declared in 1981. A Prince of Songkla University study lists its nine islands: Ko Muk, Ko Kradan, Ko Waen, Ko Chao Meng, Ko Pling, Ko Chao Mai, Ko Ngai, Ko Chueak and Ko Ma, plus Pak Meng, Chang Lang, San, Yao, Yong Ling and Chao Mai beaches and a hot spring. The park's seagrass beds feed the dugong, an endangered reserved species. Most of Ko Muk's residents are Muslim and live from fishing, rubber and coconuts.

The cave formed when limestone collapsed, so the interior resembles a crater or giant sinkhole lit through an opening above, while seawater flows in through the tunnel to create the emerald pool beneath the cliffs. The entrance is low and narrow and the middle of the tunnel is pitch dark, so a headlamp is needed.

The cave's history is tied to island life. Manager Online reports that the first locals went in to collect edible swiftlet nests, as many birds once lived there, and stories say pirates hid their loot on the secret beach before taking it away. The pirate tale is local legend without evidence.

The tsunami of 26 December 2004 also reached Ko Muk. The Showcaves website says two visitors died at the cave entrance and about 80 survivors were rescued after some five hours, though no Thai report confirming these figures has been found.

Visitors float or swim along a rope through the tunnel, about 10–15 minutes each way, only at low or mid tide: at high tide the entrance is submerged, while at the lowest tides the lagoon nearly dries and loses its colour. Waves here are stronger than elsewhere and there are many rocks, so non-swimmers, children and older visitors wear life jackets with a guide throughout. The light is best around 10:00–14:00.

Hat Chao Mai National Park closes marine tourism for about four months each year, 1 June–30 September, for the Andaman monsoon. In 2020, during COVID-19, the DNP reopened the Emerald Cave from 18 August with a cap of 600 visitors a day, letting rangers decide daily whether boats could go out, alongside underwater clean-ups and the removal of abandoned nets and traps to protect dugongs, a rare species of the Trang sea.

The cave has also made news over fee collection. In December 2023 the director of the National Parks Office made a spot check after long-standing complaints that no one collected entry fees at the cave, and found no collectors in either the morning or afternoon, while the park's revenue was abnormally low compared with neighbouring parks. The DNP set up a fact-finding committee and moved the head of Hat Chao Mai National Park to the Protected Area Management Office 5.

Hat Chao Mai has a long run of land-encroachment cases. The DNP sued the owner of land under the Anantara Si Kao resort (formerly Amari Trang Beach Resort) on Chang Lang beach, over 37 rai: the trial court dismissed the case in 2014, the appeal court ordered eviction and revocation of the title in 2015, and the Supreme Court upheld that on 25 January 2018; three luxury villas were demolished in February 2019. On Ko Kradan, claims to six plots totalling over 100 rai were found; the National Anti-Corruption Commission has returned five to the state and inspected again on 6 February 2025. On Ko Muk itself, Thailandee says the resort at Charlie Beach was dismantled after a court ruling, without giving a year.

The seas around Ko Muk also face coral bleaching. In May 2024 the park found almost 70% of the reef off Ko Kradan bleached and closed snorkelling there from 15 May 2024. Ko Kradan, on the same tour route as the cave, was named the world's best beach by the World Beach Guide in 2023 and 2024.

The gravest problem in the seas around Ko Muk is dying seagrass and vanishing dugongs. Thai PBS reported that surveys in 2023 found at least 180 dugongs in the Trang sea, including 12 mother-calf pairs, but aerial and boat surveys by the Lower Andaman marine research centre on 5–11 March 2024 around Ko Libong and Ko Muk found only 36, with a single mother-calf pair. Humpback dolphins fell from 19 to 6 and sea turtles from 174 to 38, and more than 30,000 rai of seagrass died. The Department of Marine and Coastal Resources blamed unusually low, prolonged tides linked to global warming that left seagrass baking in the sun, while villagers blamed sediment from dredging the Kantang channel.

Most visitors come on a four-island tour, usually Ko Muk, Ko Kradan, Ko Chueak and Ko Waen, from Pak Meng pier in Sikao (about 40 minutes) or from the Kantang side (Hat Yao and Khuan Tung Ku piers, about 30 minutes). A 2012 report put a full-day boat charter at about 1,500 baht. To avoid crowds, take the first tour of the day and arrive before 09:00.