Jellyfish Lake in Palau. Apparently the jellies have lost their ability to sting because of lack of predators in the lake and you can swim with them!
WAIT BUT THAT’S NOT EVEN THE COOLEST PART: These jellyfish carry small populations of algae inside their bodies and derive much of their nutrition from the sugars that the algae produce. The jellyfish follow the sun across the lake each day and rotate continuously, so that the algae are always getting maximum sunlight exposure for photosynthesis. Then at night they dive to deeper parts of the lake so the algae can absorb nitrogen. It’s one of the best examples of endosymbiosis in action and it’s KICKASS.