Sonar Revealed a Phantom Seafloor Made of Migrating Fish

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When sonar began revealing the deep ocean in the early 1940s, operators noticed something deeply strange. Their instruments sometimes showed what looked like a second seafloor hundreds of meters below the surface. Even stranger, this apparent bottom could change depth between day and night. The mystery was eventually traced to the deep scattering layer, a dense region of marine life suspended in the water column. Small fish, squid, jellyfish, and other organisms can reflect sound strongly enough to produce an echo that resembles the ocean floor. Lanternfish are especially important because their swim bladders can produce strong sonar reflections. The layer moves vertically as part of a daily migration. Many of its inhabitants stay deeper during daylight and rise toward shallower water after dark to feed, then descend again around dawn. Changes in light, including moonlight, can also affect how deep the layer appears. The discovery showed that sonar was not simply revealing the shape of the seafloor. It was also detecting enormous communities of animals living in the open ocean. Modern ocean acoustics now uses improved sonar and other sensing methods to study these layers, their migrations, and their role in ocean ecosystems. The original phantom bottom became an unexpected window into a part of the ocean that is difficult to observe directly.