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Many sea animals live deep in the sea where they can hide from predators in the dark. When it is light out, they rest in the dark water 200 to 500 meters down and at dusk they migrate to the surface to feed. So every night, starting around sunset, there is a great migration of billions of animals all over the world from the depths where they spend their days to the surface. And, every dawn, they return to the deep. It usually takes them 30 to 45 minutes to make the transit and the times they start their migration and the speed at which they move depend on the species.

This population of vertically migrating marine animals was first discovered with SONAR. Early researchers were mistaken by the strong echoes returned by this layer and thought they were seeing the bottom, wheras the bottom was actually thousands of meters deeper. The deep scattering layer can be heard in the volume reverberation following a SONAR ping. In this sound file, the bottom echo has is very clear return, while the volume reverberation sounds less distinct. Here is a plot of how the deep scattering layer looks to SONAR:

And here is a plot of a rather complicated migration showing at least 6 different animal groups migrating to the surface at different times and at different rates.

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