The deep scattering layer is an ocean layer of marine animals discovered via sonar during World War II, initially mistaken for the seafloor. Primarily composed of small mesopelagic fish like lanternfish with swim bladders, these organisms perform daily vertical migrations, rising at night to feed in shallow waters and descending during the day for safety. Global biomass estimates revised in 2007 indicate 5–10 billion tonnes of lanternfish alone.