Vertical migration by mesopelagicanimals was discovered during World War II, when sonar came into use. Sonarsoundings regularly showed a series of sound-reflecting layers, or “false bottoms.” The real bottom gives a sharp, clear echo,but these layers, collectively dubbed the
deep scattering layer (DSL),
give a soft,diffuse echo that makes a shadowy traceon sonar plots. During the day the DSLlies at depths of 300 to 500 m (1,000 to1,600 ft), but at sunset it rises to the sur-face. The depth of the DSL is clearly re