The Rock Islands are some 445 green-topped limestone islets in Palau, in the western Pacific, and the sea has bitten a notch into every one of them at the same height. The cut runs one to two metres into the rock at the waterline, so each island balloons green above a pinched stem. The whole archipelago reads as a field of mushrooms. Waves are barely responsible. The work is done by snails, urchins, sponges and microbes that graze and bore along the tide line, dissolving limestone fastest exactly where the water sits. It is even all the way round because the sea has stood at much the same level here for thousands of years. Between the islands the reef climbs to within a few metres of the surface, so a mask is all you need. You hang over gardens of hard and soft coral while clouds of silver fish part around you. Sandbars come and go on the tide, and boats time the stop to the falling water: a few hours of dry land, no shade on it, then nothing.
At a sandy channel cut by German miners a century ago, giant manta rays come in to hover over cleaning stations while wrasse pick their wings over. Divers settle on the sand a short way off and wait, because swimming at a manta only drives it away. Inside the islands are about seventy marine lakes, landlocked pools of seawater, the densest cluster of them anywhere. Each has been sealed off long enough to go its own way, and the golden jellyfish of one live nowhere but that pool. People lived out here too, for more than three thousand years, fishing the lagoon and painting the cave walls, and then left the islands entirely. The place is listed for that as much as for its reefs. The notch is best met from a kayak, which goes where the tour boats cannot. You slide in under the overhanging rims, into channels that just take the hull. The rock comes to eye level, furred with sponges and shells: the animals doing the cutting.