DSL, which Daryl/Heath mentioned in the comments for the last post, stands for Deep Scattering Layer. This layer is the mass of zooplankton, fish, and other aquatic critters that make the daily migration from deep sea to the surface to feed at night. When first discovered by Navy sonar in the 1940s, the DSL appeared to be a moving sea bottom. Sonar operators and scientists were quite baffled as to why the sea bottom appeared to move up and down depending on the time of day. Scientists soon solved the puzzle by towing nets through the layer.
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