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The Upside Down is a term for the bridge that connects two worlds. These two are Hawkins in Indiana, and the world that gives rise to the Mind Flayer, Demogorgons, and other creatures. Dustin calls this other world: The Abyss. Hence, the Upside Down is an interdimensional bridge that rips through space-time. Since it is an unstable bridge, it is held in its place by the exotic matter, the centre of which is above the Hawkins lab in the Upside Down. Why is the lab, its centre? That’s because it was created by Dr Brenner when he forced Eleven to make contact with Henry in the other realm. This contact made the wormhole connecting the two worlds through the bridge. And this bridge has helped all those monsters, and Henry himself to crossover into Hawkins.
