It's pretty hard to believe this picture is real. If you've ever been warned of the perils of swimming with jellyfish, you'd probably steer well clear of a lake infested with them. However, Jellyfish Lake in the Pacific Island of Palau is quite the tourist attraction.
The lake, known as Ongeim'l Tketau locally, contains over ten million jellyfish that evolved from one common ancestor, a spotted jellyfish that wandered in from the ocean many, many years ago. When access to the sea was cut off, the population was isolated.
Over the years, with a constant supply of algae and no predators to fend off, their stingers sort of devolved. And that's the reason it's possible to swim with them. They do still have stinging cells, but they're so small that their touch isn't detectable by human skin.
So much so that snorkelers descend on the lake regularly to attempt the bizarre feat. The jellyfish have even been featured on reality TV, with contestants on Survivor: Palau treated to a swim as a prize.
Far riskier than their stingers is the layer of hydrogen sulfide 15 to 20 meters below the surface. Humans are strongly discouraged from descending that far, and scuba diving is prohibited, due to the risk of hydrogen sulfide poisoning. (Think about swimming in sulfury, stagnant pond water, and you'll get the idea…) However, the jellyfish love it there, and that's where they spend their nights. Check out this link to see what it's like. RM