On "Stranger Things," it's fairly safe to say that the Upside Down is the source of most of the show's monsters and mayhem. Introduced at the very beginning of the show, the terrifying alternate dimension is home to deadly monsters that now want to expand their territory into our own world. We're still a little foggy on when and how the Upside Down was created, and at least one theory suspects that the characters we know were responsible for its creation.
Who Created the Upside Down?
In the "Stranger Things" timeline, the earliest we see the Upside Down is during Eleven's time in the Hawkins National Laboratory when an experiment guided by Dr. Brenner results in her accidentally opening the gateway between our world and the hellish dimension on the other side. As the fourth season reveals, though, the opening of that gateway isn't the first time Eleven encounters the Upside Down. During One's brutal attack on the lab, El defeats him by using her own powers to transport him into a dimension that we now know as the Upside Down, where he is absorbed by its power and eventually becomes Vecna.
Did the Upside Down Exist Before Eleven and One?
The mysterious origins of the Upside Down leave a lot of questions unanswered. The biggest one is whether or not it actually existed before Eleven's breakthroughs, or whether it was created by her, One, or both of them. On top of that, if they were somehow responsible for its existence, was it really an accident, or was all of this planned somehow by Brenner? Did he know (or at least suspect) the potential power of the Upside Down all along, or was its discovery a frightening but potentially powerful surprise? If the Upside Down was created by one of the lab test subjects, that could be hugely relevant as the Hawkins crew fight to defeat the monsters coming out of the dimension. After all, if it's created by humans, it could potentially also be destroyed by humans.
On the other hand, there are implications that the Upside Down has been there for a very long time, and the only "new" thing is the opening of the portal. That, too, is linked to One and Eleven, as the first known connection between the two dimensions would be Eleven banishing One to the Upside Down. El has also been the key to closing the gateway between the dimensions before, indicating that, regardless of the Upside Down's origins, she holds the key to keeping its evil out of our world.
For now, we'll have to wait and see if the rest of "Stranger Things" season four - or the upcoming final season - gives us more information on the creation of the Upside Down. There's always something new and creepy in store, and we're sure it won't disappoint!