 Sonar operators thought they might be seeing uncharted underwater islands. But scientists soon came up with another explanation. Sonar was revealing a false bottom, a layer with so many small fish and other organisms that it was reflecting the sonar. It was named the deep scattering layer. It's found in most of the world's oceans, generally at depths of 1,000 to 1,500 feet. It's part of the daily migration of the critters that live there.