Stranger Things has been about the Upside Down since the very beginning. It’s the scary, twisted version of our world that spawned all the evil that has driven the action for five seasons. Or so we thought. Finally, in the most recent episodes of the show, Stranger Things revealed exactly what the Upside Down is. And, according to the show’s creators, it’s something they’ve known about since the very first season.

In volume two of Stranger Things 5, which just debuted on Netflix, Dustin figures out that the Upside Down is not at all what they think. They’ve always assumed it was the origin of the evil, but, thanks to Dr. Brenner’s notes, Dustin realizes it’s something far worse. It’s a bridge to an alternate world they nickname “The Abyss,” which Vecna is trying to merge with ours. Everything we thought came from the Upside Down, such as the Demogorgons, actually comes from “The Abyss” and merely travels through the Upside Down to get there.

It’s a pretty massive reveal and, according to Ross and Matt Duffer, one they had in place almost 10 years ago when the show first debuted.

“That was something that I can say was early on,” Matt Duffer told Variety when asked about the reveal. “That was all the way back in season one. Netflix just wanted us to explain the mythology to them, because we were very adamant early on, ‘We don’t want to explain it in the show. We like that there’s mystery, and that there’s a lot that you don’t understand by the end of the season.’ They said, ‘That’s fine, but we would like to know.’ I think it was actually a really good exercise—we spent quite a bit of time with our writers figuring out exactly what the Upside Down was. We wrote a 20-page mythology document. It wasn’t called the Abyss at that point; it was called Dimension X, which is a Ninja Turtle reference.”

“But yeah, that’s been in there, baked in there, for a while,” Duffer concluded. “We’ve been holding those cards back for so long; it was a real relief to actually be able to show our hands here.”

Do we actually believe them? Probably to a point. The Duffers probably decided that there needed to be another, deeper layer to potentially reveal later on and kind of left it at that. Hence the Ninja Turtles reference, which also just happens to be the super generic name, “Dimension X.”

Speaking of the Upside Down, though, the Duffers also confirmed in the Variety interview that—sort of like how Will was used by Vecna to create the tunnels of Hawkins—Eleven was used by Dr. Brenner to create the Upside Down.