The Upside Down was a name given to describe a mysterious alternate dimension existing in parallel to the human world. The dimension was in some way 'created' on November 6, 1983.
Before the Upside Down formally came into existence, it was preceded by something else: a dimension of sprawling, floating rocks, populated by alien vines and a species of humanoid predators. A mysterious dormant organism, consisting entirely of silver-black particles, also inhabited the dimension.
Tears that open in the fabric of time and space are referred to as "gates" by the kids from Hawkins. Unlike the human world, the mirror-dimension was overrun with alien vines, spores and membranes, being completely devoid of human life. The alien vines, along with the Demogorgons were key members of the hive mind. The entirety of the Upside Down was essentially one gigantic organism, with all its contents and agents psychically connected via the Mind Flayer.
History[]
The Tourists[]
A small gate to the Upside Down opens inside a subway train and several Demodogs come through. While investigating the sounds made by these Demodogs, the kids from Hawkins meet the Turtles.