Every night, across every ocean, the sea's small animals — plankton, krill, lanternfish and the fish that hunt them — climb hundreds of metres toward the surface to feed, then sink back into the dark by dawn. It is the largest migration on Earth, and it happens twice a day, everywhere, mostly unseen.
Each animal is playing one simple game: stay where the light feels right. The shallows are rich in food but bright — and bright water lets visual predators see you. So an animal follows an isolume: the depth where the light matches its comfort level. As the sun sets, that comfort depth rises toward the surface and the whole layer rises with it; as the sun climbs, the isolume sinks and the layer sinks to hide.
Nothing here is scripted. Two thousand organisms each seek their own preferred light, and the dense living layer you see is what emerges. The dashed gold line is the depth the physics predicts — watch the swarm settle onto it.
The rhythm can break. Flood the surface with moonlight and the night is no longer dark enough to be safe — the layer refuses to rise, and stays trapped in the deep. Remove the predators and the reason to hide vanishes — the layer climbs to the surface and stays there. Real oceans show both: bright full-moon nights suppress the migration, and warming, oxygen-poor water is reshaping where the layer can go.
Something in the simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.