The Rock Islands are some 445 green-topped islets of limestone scattered across a turquoise lagoon south of Koror in Palau, in the western Pacific, undercut by the sea into mushroom shapes and hiding inside them the densest cluster of marine lakes on Earth. Each island sits on a pinched stem: the sea has eaten a notch one to two metres into the rock right at the waterline, so the forested top balloons out above the water like a cap. From a boat they pass in dozens, no two the same shape, with white beaches in the gaps and the water so clear that the coral is visible from the deck. A kayak gets where the tour boats cannot, under the undercut rims, into sea caves and through arches into lagoons closed off from the open water. Sandbars appear and vanish with the tide, a few hours of dry land in the middle of the lagoon and then nothing at all, and the boats time the stop to the falling water and pull off before it closes over.
Hidden inside the islands are around 70 marine lakes, landlocked pools of seawater reached by seepage through the rock, several holding animals that live nowhere else; the most famous is Jellyfish Lake, a short hike over one islet's forested spine, where millions of golden jellyfish that have lost their sting pulse slowly around you as they follow the sun across the water. At German Channel, a passage cut through the reef by German miners a century ago, giant manta rays glide in to hover over cleaning stations while small wrasse pick parasites from their wings, most reliably from December to March, the calmest and clearest months. The islands are not empty ground: caves and hillsides hold stonework villages, burial sites and rock art from people who lived here for three thousand years and then left. Palau made its waters one of the largest marine sanctuaries in the world, and the Rock Islands are the part of it you can float through in a day.