Below the sunlit zone lies the mesopelagic, or twilight zone, spanning from 200 to 1,000 meters. Here, only faint traces of sunlight penetrate, creating a dim, blue-gray world. This zone is home to one of the most remarkable phenomena on Earth: the diel vertical migration. Every day, billions of organisms — fish, squid, jellyfish, and crustaceans — ascend to feed in surface waters at night and descend to the safety of darkness at dawn. This massive movement of biomass is the largest migration on the planet.