From the FAQ on wherethehellismatt.com I learned about Jellyfish Lake. Once connected to the ocean in the distant past, the lake was cut off from it long ago and the jellyfish -suddenly without predators- lost the potency of their sting. Now they exist in astounding abundance and humans can swim right through them.
However, the lake isn’t without its dangers, as Wikipedia notes:
If there was only one place I could travel to, it would be Jellyfish Lake. Apologies to the Nederlands, France, Machu Picchu and Japan… maybe in another lifetime.