Netflix recently confirmed that Stranger Things 4 is coming. When it does make a return, it's going to have plenty of questions that need to be answered. One question surrounding the series that fans have wanted to know for quite a while now is "just what is the 'Upside Down'?"
Over the course of the first three seasons of Stranger Things, fans have caught glimpses of the mirror-type world that appears just like ours, only rotted out and desolate. Viewers have also been treated to a look at the creatures that dwell in that realm.
For most of the series, the creators of the Netflix original show have led people to believe the Upside Down is an alternate dimension. That explains why it appears so similar to our own, but also why there aren't any people to be found. Now, a new fan theory given voice by Screen Rant claims that the land could be the distant future, rather than an earth parallel to our own.
As the site points out, everything that exists in the Upside Down appears to be covered with spore-like substances, or vines and other plants that bear a resemblance to earthly plants. The laws of physics seem to exist in the same way they exist in the "real" world of Stranger Things as well.