Green Rock

Green Rock is a cluster of large granite boulders locked together off Koh Nang Yuan's north coast, riddled with caves, caverns and narrow passages that give it a "Swiss cheese" character. Dives typically descend past moray eels and the occasional grouper to the deeper boulders, reaching the site's maximum around 29 metres, where whip coral appears and visibility thins out slightly. From there the route climbs back through the trigger pit, a sandy, cratered clearing north of the main cluster where nesting triggerfish hold territory, and finishes among the swim-throughs of the main boulders. The complex does not end at the main cluster: it stretches northwest to a smaller outer pinnacle divers know as Blue Rock, its top around 26 metres; some dives fold it in as an extension, others treat it as its own stop.

Green Rock's identity rests on the trigger pit and on current, and neither is shared with its neighbours. The trigger pit is a named, mapped nesting ground where titan and yellowmargin triggerfish hold territory each spring; divers are specifically briefed on it before entering the water. Current is the other half of the case. Green Rock's north-facing position on Koh Nang Yuan exposes it in a way most of Koh Tao is not, and every operator running the site independently confirms it: real, recurring current on an island whose whole reputation is built on calm water. A shallower, easier shore-dive option exists nearby along Koh Nang Yuan's coastline for divers who want the location without the current or the depth.

Give the trigger pit space during the spring breeding season and let your guide set the approach; both nesting species will defend the nest against a diver who crowds it. The boulder crevices and swim-throughs reward tight buoyancy control, since they are an overhead and entanglement-adjacent environment separate from the site's raw depth; stay out of any passage you have not been briefed on. Visibility, already a variable 2 to 20 metres, drops further around the full moon, so plan bookings around that window with care. A torch pays off for the caves. Green Rock is conditions-dependent and sometimes skipped in favour of a site further offshore when a boat is already heading that way, so confirm it is specifically on the day's schedule if it is the reason you booked. No permit or marine-park fee applies to diving here.