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Since submarines began roaming the depths in World War I, sailors and oceanographers, who use sonar technology to map seafloor topography and identify ocean life, have regularly run into “acoustic ghosts”—inexplicable bodies of movable mass that sometimes rivaled the size of a city—writes ocean engineering professor Nicholas Makris in IEEE Spectrum(Aug. 2011). Affectionately called UFOs (unidentified floating objects), these sonar readings were blamed on factors as various as deep-ocean mountains and changes in water temperature. Every time a theory emerged to explain the phenomenon, however, it was quickly shot down.

In 2003 Makris and fellow scientists aboard a research vessel just south of Long Island, New York, discovered that the UFOs were composed of hundreds of millions of fish—massive gatherings on a scale never before documented.

Image by Wayne Levin. Article at UTNE.