At various times in the past - with sea levels fluctuating by as much as 150m with the advance and retreat of the polar ice-caps - these islands have been under water, battered by storms, covered with marine growths, visited by creatures long extinct.

Think of that when you clamber up the trail to Sailing Boat Rock, on Island No. 8, for example. As you squeeze through the crevices and archways, imagine them covered with colourful corals, sponges and algae. Where today you find birds and butterflies and squirrels, at one time dense schools, bright streams of fish instead commuted this way and that, with bigger fish and marine dinosaurs cruising through on the hunt.

A variety of forces have given shape to these islands. To begin with, the Similans were intrusions, upwellings of hot magma that found their way through weak spots in the Earth's crust 100-150 million years ago, working their way through thick layers of sedimentary rock already laid down at least 100 million years earlier still. Then, unimaginably powerful movements in the crust cracked the granite substratum into blocks, preparing the way for experiments in sculptural form by wind and wave.

Today, piles of curious stones, some of them as big as houses, lie as though collected and later abandoned in careless heaps by some ancient race of beachcombing giants.

Even Sailing Boat Rock, the distinctive formation teetering high above the cove on Koh Similan (Island No.8), has been shaped in this way. And boulders just like these spill in jumbled piles down beneath the surface of the sea to 35m and beyond, where submarine peaks, canyons, caves and passageways provide scuba divers with some of the most interesting submarine prospects in the world. (On the west side of the islands, currents have kept the formations clear of sand; on the coral-covered sandy slopes of the east side, the boulders have been largely buried.)