The photic zone refers to the portion of the water column in a lake, sea, or ocean that receives light, while the aphotic zone is the part of the water column that doesn't. Because light is required to photosynthesize, a body of water's primary productivity (the amount of biomass generated directly by the sun's energy) is directly proportional to the size of the aphotic zone. Formally, the aphotic zone begins where less than 1% of light penetrates.

In the ocean especially, the aphotic zone is a very deep area. It extends from 0.9 - 4.6 km (3,000 - 15,000 feet) in depth. A variety of bizarre animals live here, including the giant squid, vampire squid, gulper eel, angler fish, and many others. Quite a few animals in the aphotic zone are bioluminescent, that is, capable of generating their own light.

Where the aphotic zone begins depends on the number of particles suspended in the water, a quality called turbidity. It ranges significantly based on the season, whether it has rained recently, the type and location of the body of water, etc. Turbidity can be roughly measured by a simple device called a Secchi disk, which consists of a disk covered in an alternating black and white pattern. The disk is put on a rod and lowered into the water. The point at which the pattern becomes impossible to see tells us roughly how turbid the water is. For a more accurate measurement, a device called a nephelometer is used.

In the world's oceans, the aphotic zones are relatively deserted in comparison to the photic zones. Imagine if you took a gigantic sunshade and used it to enclose everything in a patch of forest. Eventually, most forms of life in the forest would die. However, in the oceans, detritus raining from above provides a source of food, so some life can survive. Other animals spend time in the aphotic zone but ascend to the photic zone to feed.