Jellyfish Lake is a pocket of seawater sealed inside a limestone island in Palau, and it holds millions of golden jellyfish that have almost entirely lost their sting. That is why you can swim out into the middle of them. The island is Eil Malk, one of the Rock Islands scattered through the lagoon south of Koror, in the western Pacific. Seawater still seeps in through cracks in the rock, but no fish that eats jellyfish ever got in, and over thousands of years the animals here gave up defences they no longer needed. They live on light instead. Algae in their tissue do the feeding and take sunlight as payment, so the whole swarm crosses the lake every day behind the sun. It stops short of the shaded rim, turning back before the shadows of the shoreline trees, because anemones that can still eat them wait in the shade.
You come to the water on foot, over a steep wooded ridge from the boat landing; nobody lives on the island, and the path and the dock are all there is of it. Then you breaststroke for the open middle, where the light is strongest and the swarm is thickest. The water is warm and completely still, so the crossing is slow work with your face down and the bells sliding past your arms, thousands in view at once and none of them doing anything at all. They are their own subspecies, found in this lake and no other. Eil Malk holds more than ten marine lakes inside its limestone, and this is the only one in Palau open to swimmers; the rest are research ground. All of it happens in the top few metres. Below about fifteen the water holds no oxygen and never mixes, and nobody dives here. In 2016 a drought warmed and salted the lake and the jellyfish stopped appearing altogether, so Palau closed it rather than sell an empty swim. The polyps on the rock walls survived, and out of them the swarm built itself back.