The Upside Down was named by team El in the first season of Stranger Things. Does it have a scientific and official name? That remains to be seen, but it's probably stuck being referred to as the Upside Down by most fans of the show for the rest of eternity.
While fans may know what to call it, what it actually is that's being referred to remains to be seen. Even more so than the powerful Eleven herself, the Upside Down has yet to have all of its mysteries uncovered. A lot of things just don't make sense when examined too closely. How did it start? Why is it here? What does it do? Here are just a few of the details that fans don't yet understand.
Its Powers
Other than the fact that it seems to breed monsters, the Upside Down doesn't seem to have any powers of its own.
Are scientists interested in it because it does have powers of its own, or only because they found it and couldn't stop? Can it teleport people? Bring back biological weapons? Offer up psychic powers? It's unclear what the Upside Down does at this point other than work as a pretty terrible prison for unfortunate preteen boys (sorry Will).
Its Age
Obviously during the 80s, when Stranger Things is set, the scientific community (and Russia) seem pretty interested in the Upside Down.
Are they interested because the realm just appeared, or simply because they just noticed it? Has the Upside Down been around for decades and only now made its presence known, or did it suddenly appear in the 1980s, and that's why attention is suddenly being paid? No one knows right now, but maybe, someday soon, both viewers and scientists will figure it out.
Who Lives There
Will was taken by the Demogorgon into the depths of the Upside Down, but it's not clear if anything besides the Demogorgon and later the Mind Flayer lives in the demon world beneath Hawkins.
Are there other life forms (evil or otherwise) that regularly inhabit the Upside Down? No one, aside from Will, seems to have spent enough time in those detestable surroundings to really figure out what does or did live there. It could just be too scary for anything to have thrived there at any point.
Its Size
It's clear that the Upside Down exists beneath Hawkins, but how big is this world overall? Does the Upside Down reach across every corner of the earth and fans have only seen the piece under Hawkins because that's where a portal has allowed access?
So far, it's unclear how far the Upside Down reaches in the world of Stranger Things. Perhaps the appearance of Hopper in Russia (in the trailer for Stranger Things season four) may offer some clues.
Why It's In Hawkins
If the Upside Down doesn't reach every corner of the earth of the 1980s, why is it in Hawkins? Did the scientists create it? It is man-made?
Without knowing how far the Upside Down reaches, it's hard to say why it happens to exist under Hawkins in particular, if that happens to be the case. Season four looks like it will answer at least a few questions about the reach of the world beneath the feet of Hawkins residents, but season four isn't quite here yet.
Entrances and Exits
Again, it's hard to say where access points are since viewers don't yet know how far the Upside Down reaches.
But, aside from the lab in Hawkins and the portals through which Will and others have slipped from Hawkins proper, does the Upside Down have other access points? Does Russia have a portal opening as well? Are there portals across the globe? Again, it remains to be seen, though season four seems to point in the direction for answers.
Who Has Visited
El has seen the Upside Down, Dr. Brenner and a few lab workers have accessed it, and of course, poor Will certainly spent more than his fair share of time there, but has anyone else visited the Upside Down?
Again, it's hard to judge without knowing how far the realm reaches or how many access points there are, but the question remains. Who has visited this world of evil and demons before? And how many have done so on purpose?
When People Have Visited
Viewers have seen El, Will, and the Hawkins lab sneak back and forth into the Upside Down during the 1980s but, along with the question of who has visited the realm is when. Did anyone know about the world's existence before the boom in 1980s Hawkins?
Has Russia been inside? Did they access the world long before the United States? Does the Upside Down even exist on the same timeline as the world above?
The Monsters
The Demogorgon and the Mind Flayer both escaped the Upside Down to terrorize Hawkins, but that doesn't mean they're necessarily from the Upside Down.
If there are other portals, it's possible they slipped into the Upside Down (on purpose or on accident) from somewhere else entirely. It's not clear if the monsters were born in the Upside Down or if they simply used it as a passageway to reach Hawkins and the world viewers recognize.
The Timeline
Whether or not the Demogorgon and Mind Flayer are originally from the Upside Down, one has to wonder how long they have existed. Did the Upside Down extend their lives or shorten them? Have they always existed along with the realm from which they later escaped, or did they suddenly pop into existence because of problematic scientists?
No one knows yet, but Stranger Things certainly has a few more seasons to explain itself.