Phi Phi, Koh Haa and the southern Andaman pinnacles

South of Phuket the geology changes from Similan granite to karst limestone — the same rock that makes the Phang Nga sea stacks and the cliffs of Phi Phi. Underwater it produces sheer walls, caverns and chimneys instead of boulder piles.

Koh Bida Nok and Koh Bida Nai are two limestone islets south of Phi Phi Leh, their walls dropping to about 25 m over sand, and they are the reliable place in Thailand to find leopard sharks lying on the bottom. Koh Haa, further south towards Koh Lanta, is a ring of five islets around a sheltered lagoon, with a large air-filled cavern known as the Cathedral and a chimney that runs from 18 m up into the rock.

Out in open water sit the two pinnacles that make the trip: Hin Daeng (Red Rock) and Hin Muang (Purple Rock), 500 m apart, 70 km from the nearest land. Hin Daeng barely breaks the surface; Hin Muang never does, which is why it took until the 1980s to be found. Hin Muang's wall is the deepest in Thailand, running past 70 m, and its purple name comes from the soft coral that covers it. Both are manta and whale-shark sites, and both carry serious current.