Deep in Palau's Rock Islands is a remarkable marine lake where millions of golden jellyfish swim daily. The golden jellyfish have evolved to be distinctly different from their ocean-dwelling relatives. Their stings are extremely mild and generally harmless to humans.
Each morning, the jellyfish move across the lake, then reverse course in the early afternoon. You can see this ethereal scene here, where a lucky person gets to swim alongside the jellies. This looks like it could easily be the best experience of one's life.
While tourists can snorkel with the jellies, scuba diving is prohibited both to protect the creatures and because the toxic lower layer can be absorbed through divers' skin. "The jellyfish play an important role in mixing the water and its content of oxygen and nutrients in the lake," according to research cited in Wikipedia, highlighting how these wonderful little creatures help maintain their own unique ecosystem.
See also: Why don't jellyfish sting each other?
