There's a curious feature at the bottom of the Gulf of Mexico. It's a lake of sorts, which has been referred to as the Jacuzzi of Despair, as animals that enter it will not emerge.
The water in the 'lake within the sea' is about five times as salty as the water surrounding it. It also contains highly toxic concentrations of methane and hydrogen sulphide and can thus not mix with the surrounding sea.
Yet microbes, tube worms, and other forms of life call it home.