Green Rock: Koh Nang Yuan's Granite Maze Dive Site

Green Rock is Koh Tao's granite maze. Out on the exposed north-west side of Koh Nang Yuan, a big stack of boulders has cracked and settled into swim-throughs, canyons and short tunnels, and threading through them is the whole point of the dive. If you are looking for a Green Rock dive site on Koh Tao that feels more like exploring than sightseeing, this is it.

It is not a beginner site. The rock tops sit around 5 to 6 m, the base of the boulders runs to roughly 10 to 20 m, and the open water side of Nang Yuan gets more current than the sheltered bays. Come with decent buoyancy, an Advanced certification if you can, and a guide who knows the route. In return you get one of the most interesting bits of topography around Koh Tao, plus a patch of sand nearby where triggerfish dig their nests.

Green Rock dive site map

1. 1Koh Nang Yuan, north-west coast
2. 2Nang Yuan coastline reef, 7 to 12 m
3. 3Main boulder cluster, tops 5 m
4. 4Trigger pits, sand about 18 m
5. 5Blue Rock pinnacle, top 26 m
6. 6Approximate mooring

Diving Green Rock: the route

Most dives start on top of the boulders and work down into the passages. Some gaps are wide enough for a few divers at a time, others for one diver at a time, so groups tend to string out and take turns. If a passage looks too tight, you can simply follow the outside base of the rocks and rejoin the group on the other side.

North of the boulders is a flat patch of sand called the trigger pits. Your guide will take you past it, but only at a distance, because titan and yellowmargin triggerfish dig their nests there. From the pits, stronger divers can carry on north-west to Blue Rock, a small separate pinnacle in deeper water. The dive usually finishes by drifting up along the shallower Nang Yuan coastline, which makes a gentle ascent and a relaxed safety stop.

Underwater layout

The main body is a cluster of large granite boulders. The tops come up to about 5 to 6 m below the surface and the base lies somewhere between 10 and 20 m, with the average dive spent around 16 to 20 m. The gaps between boulders form caves, caverns, canyons and swim-throughs of different sizes.

• Boulder cluster: the heart of the site, full of passages and overhead rock.
• Trigger pits: a sand flat just north of the rocks, with nesting hollows.
• Blue Rock: a lone pinnacle further north-west with its top near 26 m and whip corals on its flanks.
• Nang Yuan coastline: a shallower reef at around 7 to 12 m, used for the ascent or as an easier alternative dive.

Overall the site reaches about 28 m. Depths shift a little depending on the tide and where you measure.

Marine life at Green Rock

Titan and yellowmargin triggerfish are the stars of the sand, and you will usually see butterflyfish, angelfish, longfin bannerfish, white-eyed moray eels and harlequin sweetlips around the rocks. Yellowtail barracuda hang near the tops of the boulders. Hawksbill and green turtles come through regularly, and a whale shark is possible in season, though never guaranteed.

Bring a torch. The cave walls hide Durban dancing shrimp, ghost shrimp, cleaner shrimp, banded boxer shrimp, pipefish and nudibranchs, and the whip corals at Blue Rock carry xeno crabs. Chevron barracuda, Jenkins whiprays and banded sea kraits turn up now and then.

Conditions and best time to dive

Green Rock faces open water, so the current can run mild to strong. Visibility swings a lot, from around 5 to 15 m on a typical day to 20 m or more when things line up, and it is usually worse around the full moon. Conditions are generally kinder outside the June to August south-west monsoon months.

April and May are the peak of triggerfish nesting, which makes the trigger pits busier and the fish more protective. It is still a good time to dive here, you just need to give them more room.

Tips from local divers

• Sort out your buoyancy before the narrow passages. Fin kicks stir up silt and ruin the view for everyone behind you.
• Take a torch for the cave walls and cracks.
• If current is running, choose the shallower Nang Yuan coastline for a gentler dive.
• Check conditions around the full moon, when current and visibility are often at their worst.
• Stay well back from the trigger pits, especially in April and May.

Safety notes

Depth reaches about 28 m, so you need a deep qualification for anything below 18 m. Overhead rock in the swim-throughs means good buoyancy is not optional. Triggerfish are territorial during breeding season, roughly April to June, and will charge divers who stray too close so keep your distance and follow your guide's lead. Current can be strong on this side of the island, so listen carefully to your briefing.

Why dive Green Rock

• Granite maze of swim-throughs, caverns and canyons
• Trigger pits with nesting titan and yellowmargin triggerfish
• Blue Rock pinnacle at about 26 m
• Regular hawksbill and green turtle visits
• Yellowtail barracuda near the top of the rocks
• Whale sharks pass through in season

Green Rock: frequently asked questions

Nearby dive sites

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