During WWII, naval oceanographers discovered a reflective layer that rose and fell across their sonar screens once each day. Further research revealed that it comprised swarms of fish and tiny sea creatures called zooplankton migrating toward the ocean surface as the sun set to feed under cover of darkness, then swimming back to the inky depths at dawn to escape their own predators during daylight hours.
Click here for original story, Scientists use satellite-mounted laser to map global ocean migration
Source: Phys.org