On one of the uninhabited pacific islands of Palau, there is a lake. Once upon a time this lake was connected to the sea. This connection closed and any predators still in the lake died soon after leaving native jellyfish isolated. The jellyfish stingers devolved to the point where they don't sting human skin, making this a popular dive spot.
Don't dive too deep; there's a layer of hydrogen sulfide in the lake 15-20 meters deep. Drink it and you die.