A journey of several hours into the bottom of the ocean takes you into the deep sea where light cannot reach. In the pitch black, the temperature is near freezing. But sometimes in the darkness there are occasional brief flashes of light. This is bioluminescence, produced by the creatures that inhabit the shadowy realm of the deep sea.
These creatures must survive in extremely harsh conditions, with small amounts of oxygen, very little food, no sunlight, and constant, extreme cold. Most creatures have to depend on food floating from above. For this reason scientists once assumed that life would be sparse in the deep ocean but virtually every probe has revealed that, on the contrary, life is abundant in the deep ocean.