In the dark waters below the sunlit zone, many organisms make their own light. Bioluminescence is the chemical production of light by living creatures, achieved through reactions involving luciferins and luciferase enzymes or through symbiotic bacteria.

The light can serve as a lure for prey, a warning signal, camouflage against the downwelling light, or a means of communication. Deep-sea fishes, jellyfishes, squid, and countless planktonic organisms flash, glow, or release luminous clouds. In surface waters, dinoflagellate blooms can make the waves sparkle at night.

The phenomenon is far more common in the ocean than on land. Studying bioluminescence has yielded tools for biomedical research and inspired new imaging methods. Divers and submersible pilots often describe the experience of stimulating a field of glowing plankton as one of the most memorable sights in the sea.

A single net haul from the deep can contain dozens of luminous species.