The deep scattering layer, sometimes referred to as the sound scattering layer, is a layer in the ocean consisting of a variety of marine animals. It was discovered through the use of sonar, as ships found a layer that scattered the sound and was thus sometimes mistaken for the seabed. For this reason it is sometimes called the false bottom or phantom bottom. It can be seen to rise and fall each day in keeping with diel vertical migration.
Sonar operators, using the newly developed sonar technology during World War II, were puzzled by what appeared to be a false sea floor 300–500 metres (980–1,640 ft) deep at day, and less deep at night. Initially, this mysterious phenomenon was called the ECR layer using the initials of its discoverers. It turned out to be due to millions of marine organisms, most particularly small mesopelagic fish, with swim bladders that reflected the sonar. These organisms migrate up into shallower water at dusk to feed on plankton. The layer is deeper when the moon is out, and can become shallower when clouds pass over the moon. Lanternfish account for much of the biomass responsible for the deep scattering layer of the world's oceans. Sonar reflects off the millions of lanternfish swim bladders, giving the appearance of a false bottom.
The phantom bottom is caused by the sonar misinterpreting as the ocean floor a layer of small seagoing creatures that congregate between 1,000 and 1,500 feet (300 and 460 m) below the surface. The name is derived from the fact that the first people to see these measurements erroneously reported that they had discovered sunken islands. Most mesopelagic fishes are small filter feeders which ascend at night to feed in the nutrient rich waters of the epipelagic zone. During the day, they return to the dark, cold, oxygen deficient waters of the mesopelagic where they are relatively safe from predators.