Everything You Need to Know About the Upside Down, the 'Stranger Things' Parallel Universe

“The Vale of Shadows is a dimension that is a dark reflection or echo of our world. It is a place of decay and death. A plane out of phase. A place of monsters. It is right next to you and you don't even see it.”

Stranger Things
, Chapter 5: “The Flea and the Acrobat”

Imagine a world exactly like our own, but distorted by time and space. You can still access familiar places, but they're askew, empty; they lack the principle of formality, and appear as mere simulations. This is, in essence, the Upside Down of Stranger Things, another dimension that is presented to us as a membrane-dripping, shadowy version of the world we know and inhabit.
 The first season of Stranger Things keeps the Upside Down shrouded in mystery beyond being an alternate dimension that resembles our own. It is inhabited by a humanoid monster (nicknamed the Demogorgon) that feeds on eggs and abducts children. It is dark, blue-hued, and can, to the best of our knowledge, only be accessed by people with telekinetic properties or through energy-induced portals.
 Stranger Things returns to Netflix with Season 2 (and a clear promise of Season 3), and if the trailers are any indication, the line between our world and the Upside Down will be flimsier than ever before. Is Will Byers experiencing visions of that world, or was his return from the Upside Down a dangerous way of binding our dimensions, perhaps detrimentally?