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. Every time a theory emerged to explain the phenomenon, however, it was quickly shot down.

In 2003 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. Using low-­frequency sonar technology that penetrated hundreds of miles, they identified a school roughly the size of Manhattan.

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