During the 1940s, echo sounders repeatedly returned a strong reflection several hundred metres down in deep water, well above the true bottom. The layer moved upward at dusk and downward at dawn, which no sea floor does, and it became known as the deep scattering layer.
The reflection comes largely from gas-filled swim bladders in small mesopelagic fishes, with lanternfishes among the most numerous contributors, alongside crustaceans and siphonophores. The nightly rise is the largest animal migration on the planet by biomass, a daily movement between deep refuge and richer surface feeding.
How much fish biomass the layer actually holds has been revised upward and remains contested. Acoustic surveys in the 2010s suggested global mesopelagic fish biomass might be an order of magnitude greater than earlier net-based estimates, but nets and sonar disagree because fish avoid nets and because acoustic conversion depends on assumptions about swim bladders.