Koh Ha Yai

Koh Haa Yai is the largest island in the Koh Haa archipelago, best known for a dramatic underwater cave complex called the Cathedral. The south side features a broad coral reef plateau leading into two large limestone caverns connected by a wide swim-through, with sunlight illuminating stalactites and coral- and sponge-covered walls. Outside the caves the reef slopes and pinnacles extend to about 30 m and support plentiful hard and soft corals and compact reef life; marine life is rich, with frequent sightings of hawksbill turtles and reef sharks. Dives are boat-access only from a moored dive boat, with typical depths about 3-30 m depending on route and a usual descent to a shallow ledge or plateau around 5-10 m. Visibility often ranges 15-30 m and water temperature is about 27-30°C. Currents are generally mild but can run moderate on the reef edge, so drift dives are possible; the first cave entrance is near 18 m and is narrow, so carrying a torch is recommended and careful buoyancy control is important when ascending out of the caverns. Common routes start on the shallow plateau, pass into the Cathedral, then continue along the eastern sloping wall toward 25-30 m with a common exit at a small rocky bay on the southeast tip.