A layer of organisms found in most oceanic waters that scatters sound. These layers are usually found during the day at depths ranging from 600 to 2400 feet, are rarely less than 150 feet thick, and can be as thick as 600 feet. Several layers are often recorded simultaneously and can range horizontally for many kilometers. Most of these layers undergo diurnal vertical movements. There are also shallow (over continental shelves) and surface scattering layers.