Nine years later, fans are finally getting answers to long-awaited questions regarding the Stranger Things universe — including what the Upside Down actually is.

The revelatory moment arrives during Stranger Things Season 5, Volume 2, when Dustin Henderson (Gaten Matarazzo) comes to the chilling realization that everything they thought they knew about the Upside Down has been “dead wrong.” It’s long been thought to be an alternate dimension — except it isn’t one at all.

“We’ve slowly been peeling back the layers over the seasons, but in our final season, we wanted to explain finally what the Upside Down was,” cocreator Ross Duffer recently told Netflix’s Tudum.

So what exactly is the Upside Down? What prompts Dustin’s revelation? And does this actually change things for the Hawkins gang? Below, we’re unraveling everything there is to know about the shadow dimension ahead of the Stranger Things series finale.

What does Dustin learn about the Upside Down?

In short, Dustin learns that the Upside Down has been a wormhole to an alternate dimension this entire time — not an alternate dimension itself.

“We’ve known it was a wormhole since Season 1, but it’s one thing to say it, and it’s another to try to figure out how to visualize such an abstract concept,” Duffer told Deadline.

How does Dustin come to this realization?

Dustin comes across Dr. Brenner’s (Matthew Modine) journals in the Upside Down, which contain musings about the shadowy dimension — including an analysis of the gigantic sphere of exotic matter suspended above Hawkins lab. That exotic matter, as Dustin explains, is the only thing keeping the wormhole intact; Vecna didn’t create that sphere of exotic matter — science did. Dustin realizes this just as Nancy (Natalia Dyer) shoots at it in Chapter Five: “Shock Jock.” The gunshot disturbs the sphere — though not enough to destroy the Upside Down.

When Dustin presents his findings to the rest of the group in Chapter Seven: “The Bridge,” he draws a cylinder with a circle at either end to signify how the Upside Down connects Hawkins to an alternate dimension known as the Abyss.

“It’s a wormhole,” Dustin tells Steve (Joe Keery) of the Upside Down in Chapter Five. “A bridge between two points in time and space.”

But what is the Abyss? And how does it factor into this?

While the Upside Down isn’t an alternate dimension, the Abyss is. Before she and Holly (Nell Fisher) part ways, Max (Sadie Sink) assures her that she’ll end up back in the Upside Down and that she should seek refuge in her house until they come find her — except Max is wrong. Holly isn’t actually in the Upside Down — she’s in the Abyss.

Walk with us here. The Abyss is where Holly and the 11 other children captured by Vecna are entombed in the unsightly roots of the Pain Tree. The Abyss is essentially an eerie alternate dimension characterized by desolate landscapes and scorched orange skies. The Pain Tree serves as the dimension’s landmark structure.

The Abyss is connected to the human world via the Upside Down, which we now know to be an extra-dimensional bridge between both universes.

How does this revelation change things?

Knowing that the Upside Down is essentially an interdimensional breach rather than a twisted parallel dimension is crucial to destroying it. With this newfound information, the Hawkins gang can devise a concrete plan to destroy the Upside Down and banish Vecna and the monsters for good. They can finally sever Vecna’s link to the human world once and for all.

What have the Duffer brothers said about this revelation?

The Duffer brothers, according to cocreator Matt Duffer, drew inspiration from a 1957 war film when crafting the Upside Down lore. With their inspiration finally set, the pieces began to fall into place.

“We love the film Bridge on the River Kwai,” Duffer told Netflix’s Tudum of the David Lean-directed film. “The idea in that film is they need to blow up this bridge. It’s a major military goal. So we thought the idea of a supernatural version of that, where … our characters ultimately need to blow up this ‘bridge’ that connects Hawkins to an evil dimension was a really cool goal and something we hadn’t seen before. That was something we’d been working toward for a couple years.”