A horseshoe-shaped limestone pinnacle in Mu Ko Surin National Park, roughly 18 km east of Ko Surin Tai and 45 km off Thailand's Andaman coast, widely rated the country's #1 dive site for its purple soft-coral walls, dense macro life, and seasonal whale shark and manta ray encounters from February to April.
Destination info
Conditions, highlights, and the resident marine life.
Conditions
Water and air temperature across the year.
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Description
Richelieu Rock is a solitary limestone pinnacle that rises from a sea floor of roughly 30–35 m to within 1–3 m of the surface at low tide, sitting in open water about 18 km southeast of Ko Surin Tai and 45 km off the mainland in Mu Ko Surin National Park. The reef forms a rough horseshoe of pinnacles, overhangs, and small caves — a central pinnacle ringed by smaller jutting rocks, dropping most steeply on the north and west sides while the south slopes more gently inside the horseshoe. Because the currents that sweep the pinnacle leave little room for hard coral, the rock is instead carpeted in dense soft corals, anemones, barrel sponges, and sea fans in purple, pink, orange, and yellow — the violet coloration the site is famous for. The same nutrient flow makes it one of the richest single dives in Thailand: tigertail seahorses and ghost pipefish cling to the soft corals, harlequin shrimp and frogfish hide among the sea fans, while chevron barracuda form silvery tornadoes and trevallies hunt in the blue. From February to April, plankton blooms draw the headline acts — whale sharks and manta rays, the latter visiting cleaning stations on the eastern side. The whole park sits within a strict seasonal window: it opens around 15 October and closes 16 May to 14 October each year during the southwest monsoon, so all diving is confined to the dry season. The site is large enough to warrant multiple dives and is reached mainly by liveaboard, with seasonal speedboat day trips from Khao Lak.
Highlights
What makes this dive worth the trip.
Richelieu Rock is consistently rated Thailand's #1 dive site and ranks among the best in Asia and the world — a single horseshoe-shaped pinnacle whose strong currents starve hard coral but feed dense soft-coral growth, making it one of the richest single dives in the Andaman Sea.
From February to April, peak plankton blooms draw the site's headline pelagics: whale sharks (Rhincodon typus) and manta rays, with mantas visiting cleaning stations on the eastern side of the rock. The same plankton can drop visibility while the animals are present.
The pinnacle is famous for purple, pink, orange, and yellow soft corals, anemones, barrel sponges, and sea fans, and for exceptional macro life — tigertail seahorses, ghost pipefish, harlequin shrimp, frogfish, and mating cuttlefish all on one site.
Marine life
31 species you’re likely to encounter on a dive here.
Dive sites
5 signature sites at this destination.
Richelieu Rock
The headline horseshoe-shaped limestone pinnacle, rising from about 30–35 m to within 1–3 m of the surface at low tide. The structure is a central pinnacle ringed by smaller rocks, overhangs, and small caves, dropping most steeply on the north and west walls while the south side slopes gently inside the horseshoe; the eastern side holds manta cleaning stations. Currents can be strong and shift around the rock, but it is large enough to shelter on the lee side, and the soft-coral cover — purple, pink, orange, and yellow — is among the densest in Thailand. The site is large enough to need several dives and is the reliable February-to-April spot for whale sharks and mantas.
5–35 mintermediateLiveaboardModerateVisibility 10–30 m
Koh Bon Ridge
A west-facing limestone ridge extending underwater off the western tip of Koh Bon, descending from the surface to around 35–40 m and draped in pastel soft corals. It is widely regarded as one of the best places in the Andaman Sea to see manta rays, which cruise the ridge and the deeper water to the south; leopard sharks rest on the deeper sand. The gentle wall plus schooling sweetlips and snappers make it a rewarding multi-level dive. Reached by liveaboard or seasonal day boat as part of the Surin/Similan circuit.
5–40 mintermediateLiveaboardModerateVisibility 15–30 m
Koh Tachai Pinnacle (Twin Peaks)
A large rocky pinnacle off Koh Tachai whose dome-shaped top is covered in hard coral at about 12–14 m, surrounded by boulders dropping to a sandy bottom near 30 m, with a second pinnacle across a sandy patch at roughly 25 m. One of the more current-exposed sites in the region — divers descend on a line and shelter among the boulders when the flow is strong — but it rewards with spectacular schooling fish and a reputation as a sanctuary for manta rays and the occasional whale shark from January to May. Leopard sharks rest in the deeper sand. Speedboat day trips here have been restricted since 2017, so it is dived mainly by liveaboard.
12–30 madvancedLiveaboardStrongVisibility 15–30 m
Koh Torinla
A sheltered reef off the small islet of Koh Torinla, just south of Ko Surin Tai, whose eastern reef holds some of the best hard coral in Thailand — staghorn and other species sloping gently from the beach down to about 15 m. The healthy shallow coral makes it an easy, current-light dive suited to all levels, and one of the few Surin sites where blacktip reef sharks (absent at the Similans) are regular. Hawksbill turtles are common in the shallows, alongside bumphead parrotfish, batfish, and Jenkins whiprays.
5–18 mbeginnerLiveaboardLightVisibility 10–25 m
Hin Kong (Hin Gong)
A sloping granite-boulder reef in the Surin Islands where giant boulders stack from above the surface down to a sandy bottom, broken by sandy channels and small bommies and covered in extensive hard-coral gardens. The varied topography and gentle slope make it an accessible reef and macro dive away from the more exposed pinnacles, with schooling reef fish, moray eels, and occasional turtles. Dived as part of the Surin liveaboard or day-boat circuit during the open season.
5–25 mbeginnerLiveaboardLightVisibility 10–25 m
Where to dive & stay
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