Easily the scariest part of the show was the Upside Down. This was a bleak, terrifying universe that characters traveled to in order to save the young Will Byers. Of course, Will didn't go there of his own accord.
The Upside Down had legit monsters in it. Just look at this freaking thing. It entered our world and kidnapped children. Many were even killed by the "Demogorgon," and it took the full force of Eleven's psychic powers to stop it.
Of course, Eleven was the one who initially opened the portal to the Upside Down. She made first contact and let the monsters from their world into ours. It wasn't until she sacrificed herself that the door was shut...for now.
Will may be a living portal to the Upside Down. Both of these worlds may be combining. Could this be the season that we finally learn exactly what the Upside Down is? Until it's officially explained, there's a compelling theory that may just explain what's going on.
One theory posits that the Upside Down is part of a multiverse. This is the idea that there are an infinite number of possible realities, all stacked on top of each other. Eleven opened a door to a neighboring reality, and that's what we've witnessed.
Delving a little further into multiverse theory, these different realities are all based off of slight changes in each scenario of the world. It's like the butterfly effect. A tiny action can lead to great consequences. Therefore, the Upside Down is essentially our world, but where something went terribly wrong.
What could have lead to such a disastrous fate for the Earth that hosts the Upside Down? Could the military have been responsible? After all, their experiments lead to Eleven honing her abilities and opening the door. One more wrinkle of this Stranger Things theory must be explained to fully understand what the Upside Down may really be.
Not only is the Upside Down an alternate version of our world, but it is also set in the future. Essentially, the military messed up bigly, time has passed and we are now entering the world that is left. A world full of murderous monstrosities. Luckily, our worlds are separated.
At least...they were. It now appears that Will is acting as a conduit, combining both of these realities. Season one was about kids saving their friend. Season two appears to be about kids saving the world as we know it.