Twelve thousand years ago jellyfish became trapped in a natural basin on the island when the ocean receded.
Because there were no predators of the jellyfish for thousands of years, they evolved into a new species that lost most stinging ability because they no longer had to protect themselves. Now, they are pretty much harmless to humans although some people with very sensitive skin may get a minor irritation from them.
Snorkeling near the surface of Jellyfish Lake is a popular activity for tourists in Palau. Scuba diving is not allowed because the bubbles are harmful to the jellyfish, and because diving into the lower depths of the lake is very dangerous. While the top layer of the lake is oxygenated, the lower part, which starts about 49 feet down, is anoxic (without oxygen) and contains a bacteria that absorbs all light. Additionally, the deeper water is very high in hydrogen sulfide, a toxic gas that divers can absorb through their skin.