The Demogorgon's lair is in a space known as "the Upside Down," which looks almost identical to our reality, only darker, mistier, and full of really gross goo (not even Chip and Joanna Gaines could fix this mess). Eleven confirms that this is where Will and Barb are likely being kept, which frustrates Mike, Dustin, and Lucas since they don't believe her. We also discover more about her life at the lab through disturbing flashbacks. She'd been raised there and experimented on by Dr. Brenner (Matthew Modine), who was a sort of father figure to her while also pushing her powers to their limits (and essentially forcing her to act out and kill some of the lab's employees). She spent the first years of her life in a cell, cut off from a normal childhood. Fortunately, the boys take pity on her and let her hide out in Mike's basement, since his parents never go down there.
