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:
At night, the jellyfish descend into a layer of hydrogen sulfide which is found below 15-20m of depth. SCUBA diving in the lake is prohibited to avoid disturbing the jellyfish and also to reduce the risk of hydrogen sulfide poisoning.You can also find scores of beautiful photos of the lake.
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.