5010

By the time of Monsters, Inc. though, the monsters realize that human reaction is a powerful source of energy needed to keep their world running, and that wiping them out was actually a big mistake.  So the creatures invent machines that allow an active door to transport them back in time, so they can go to a point in history when humans are still alive.  The monsters then harvest their screams for energy, and return back to the present by exiting the active door.  This system is very productive and allows the creatures to harvest enough power to support their rapidly expanding cities.

MONSTERS, INC