Phytoplankton are tiny, plant-like organisms that live in the ocean and are, basically, at the very bottom of the food chain. But, sometimes, they get their revenge. When lots and lots and lots of phytoplankton get together, they can form what we call a "red tide," a discoloration of the water at a particular point where the plankton have become densely concentrated.
Some red tides are natural. Others happen when nutrient runoff from farm fertilizers creates a massive buffet for plankton. Some red tides can kill, as the plankton can produce toxins and their deaths reduce the oxygen content of the water. And sometimes, red tides glow in the dark.
The phytoplankton in this red tide off a California beach are bioluminescent. Their cells produce a chemical reaction that creates a soft, blue-green glow. It's basically the same thing that makes lightning bugs light. In this video by Loghan Call and Man's Best Media, you can see plankton light up in the beach (and a few surfers).
Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.