Phang Nga Bay is a shallow jade-green sea between Phuket and the Thai mainland, hundreds of limestone towers rising sheer out of it, and the best of them are hollow. They are all that is left of one limestone platform, hollowed from inside by slightly acid rainwater working down the cracks. Where a cave roof later fell in, what stands is a basin walled all round with nothing over it but sky. A room like that is a hong, still tied to the sea by a tunnel at water level. A sea canoe gets you through at low tide, sometimes flat on your back to get under the roof, and comes up inside sheer walls, mangroves on the floor, birds overhead. A few hours later the mouth is under water and the room is shut. That is why your guide watches the tide, not the clock.
The long-tails run out into the open bay, engine noise banging back off the cliffs. Ahead the mangrove channels open, where the fish, prawns and crabs that feed the bay are raised and otters swim between the islets. On the cliff at Khao Khian, ochre outlines of people, boats and elephants cross the rock, left three thousand years ago and more by people already living off these tides. Ko Panyi is a fishing village on posts over the water at the foot of a cliff, walkways of planking between houses, shops and a gold-domed mosque. With no ground to play on, its children built a football pitch on a raft of scrap timber in the 1980s; the team it produced won regional titles. Ko Ta Pu, the thin rock everyone photographs, has had no boat near it since 1998 because its base is undercut, so you look from the beach next door. Before dawn from the hilltop at Samet Nangshe the headlands come out of the mist one behind another, and from there you cannot tell which towers are hollow. Only the tide can tell you that, and only for a few hours at a time.