Jellyfish Lake is a marine lake sealed inside a limestone island in Palau, in the western Pacific, where millions of golden jellyfish live cut off from the sea and have lost almost all of their sting, so that you can swim out into the middle of the swarm with nothing but a mask and fins. The lake sits on Eil Malk, one of the Rock Islands, a scatter of forested limestone humps in a turquoise lagoon south of Koror; sea water seeps into it through cracks in the rock, but no fish that eat jellyfish ever got in, and over thousands of years the jellyfish here gave up the defences they no longer needed. They live on light. Each day the swarm follows the sun across the lake, and it stops just short of the shaded edges, turning back before it reaches the shadows cast by the shoreline trees, because in the shadows wait the anemones that can still eat them.
You reach the lake on foot, over a steep ridge from the boat landing, and the last thing you see before the water is the sign that says no tanks, no fins with hard edges, no sunscreen. Then you slip in and breaststroke for the open middle, where the light is strongest and the swarm is thickest, and within a few strokes they are everywhere: soft golden bells the size of a fist pulsing against your arms and face, thousands in view at once, harmless. Eil Malk holds more than ten marine lakes inside its limestone, each with its own animals, and this is the one open to visitors. In 2016 a drought warmed and salted the water and the golden jellyfish simply stopped appearing; the lake was closed, the population rebuilt itself from tiny polyps on the bottom, and it reopened in 2019. The swarm is back, and it still turns around at the edge of the shade.