Millions of jellyfish swim in one lake so isolated they lost most of their sting

Palau's Jellyfish Lake is a marine lake cut off from the open ocean for roughly 10,000 years, home to millions of golden jellyfish that migrate across it daily following the sun. With almost no predators inside the lake, the jellyfish evolved a much weaker sting than their ocean-dwelling relatives, letting snorkelers swim among them with only the mildest of stings.

— Michael N. Dawson, Laura E. Martin, and Lisa K. Penland, Jellyfish swarms, tourists, and the Christ-child — Hydrobiologia, 2001