deep scattering layer

Etymology
From being a layer of the ocean that lies deep, and scatters sonar signals.

Noun

 * 1)  A layer of the ocean lying usually 300-500 metres deep composed of dense concentrations of pelagic animal life, that moves depending on the time of day, up at night and down during the day. It sometimes appear as if it were the seabed on sonar scopes, due to the density of life in the layer.