Jellyfish Lake is a well-known dive site in the Pacific island of Palau. It is one of the rock islands, a series of small, rocky, uninhabited archipelagos off the coast of Koror. Jellyfish Lake is completely isolated, but in the distant past it had an outlet to the ocean. The outlet was closed off and the high jellyfish population was isolated and started to feed on quickly-reproducing algae. Over millions of years, it became an advantage for the jellyfish to lose their stinging cells, or nematocysts. Today, the very high jellyfish population are stingless, and tourists can enjoy swimming with them much closer than would be possible anywhere else.

At night the jellyfish descend into a layer of hydrogen sulfide which is found below 15-20m of depth. SCUBA diving in the lake is prohibited to avoid disturbing the jellyfish and also to reduce the risk of hydrogen sulfide poisoning.