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The deep scattering layer, as it came to be known, is in fact created by sound waves bouncing off the gas-filled swim bladders of twilight zone fish and the bodies of other migrating animals, such as the relatives of jellyfish called siphonophores.

Early recordings of diel migration date to World War II, when ships and submarines using sonar to sweep the oceans for enemy subs detected something odd—parts of the seafloor seemed to be moving up and down, creating a deep “scattering layer” that reflected the sonar signals.

“From these preliminary observations there appears to be some direct correlation of the planktonic animals with the scattering layer,” Johnson wrote.

This phenomenon we now call the deep scattering layer is made up of millions of small fish, but when viewed from 1951 its first detection retains enough proximity and mystery to send a shiver up one's spine.

Scripps Institution of Oceanography scientist Martin Johnson proposed an explanation: The deep scattering layer could be marine animals migrating up to the surface.