while travelling in thailand, i was able to observe the fascinating phenomenon that is called bioluminescence:
bioluminescence is caused by a small group of algae (dinoflagellates) in marine plankton.
in response to mechanical stress (motion in the water), they are able to produce weak flickers of blue light.
according to wikipedia, it may be a way of self-defense:
When a predator of plankton is sensed through motion in the water, the dinoflagellate luminesces. This in turn attracts even larger predators which will consume the would-be predator of the dinoflagellate.
however, nothing you can read about it comes close to seeing it in the wild, ideally on a tropical island.
after a few minutes trying to find out whether my eyesight had gone completely bonkers, i spent a good hour whirling my arms and feet around in the water, swimming forth and back in an ocean of blue sparks, and, eventually, trying to capture it on photos (which didn’t prove too successful, due to the low level of bioluminescent light).
one week later, i experienced the beauty of sparkling plankton one more time, during a night dive. at one point, everybody blocked the light of their flashlights, and we followed the divemaster around, just keeping track of his flickering bluish traces.