Throughout Stranger Things, the Hawkins crew believed the Upside Down was an alternate dimension mirroring their small Indiana town. But Stranger Things 5, Volume 2 — now streaming on Netflix — flips that theory on its head.

“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,” series co-creator Ross Duffer tells Netflix.

So what is the Upside Down?

In Season 1, the Upside Down is revealed to be a parallel version of Hawkins, inundated with shadows, living vines that feed on anyone in their path, terrifying monsters such as the Demogorgons, and ashlike particles called Upside Down spores. This alternate world can be accessed through portals, or gates, which are hidden throughout the town. In the first season, the primary gate is located in the basement of the Hawkins National Lab, and government officials aim to weaponize the monsters residing in the shadow dimension. 

Eleven (Millie Bobby Brown) is the first character to call this place the Upside Down. In Season 1, Episode 5, she coins the name when Dustin (Gaten Matarazzo), Lucas (Caleb McLaughlin), and Mike (Finn Wolfhard) debate what Will (Noah Schnapp) meant when he described where he was taken as “like home, but it’s so dark.”

After reading about the Vale of Shadows — a “dark reflection or echo of our world” — the boys determine that the Upside Down must be a dimension that’s like Hawkins but darker.

The series has continued to pull back the layers of the Upside Down’s mythology.

What happens in Season 2?

In Season 2, Dustin finds a slug that he names D’Artagnan, or Dart for short. Dart’s growth spurts reveal he’s a mini Demogorgon that loves 3 Musketeers candy bars and is capable of showing affection. 

Plus, we learn that Will is connected to the shadow monster that Mike and Dustin dub the Mind Flayer (after the Dungeons & Dragon character) and that its minions are the Demodogs, canine versions of the Demogorgon. To sever Will’s connection to the Mind Flayer, Eleven seals the Hawkins Lab gate at the end of the season. 

How do the Russians get involved?

Season 3 reveals that the Russians have discovered the Upside Down and are attempting to weaponize it in their secret lab beneath the Starcourt Mall. In more unsettling news, the Mind Flayer particles that were booted from Will’s body in Season 2 never made it back to the Upside Down, and the shadow monster has now possessed Max’s (Sadie Sink) older stepbrother, Billy (Dacre Montgomery). 

While the kids fight off the Mind Flayer, the adults — including Hopper (David Harbour), Joyce (Winona Ryder), and Murray (Brett Gelman) — destroy the Russians’ weapon that would open a gate into the Upside Down, cutting off their access. But the Russians are harboring Demogorgons as pets, which can’t survive without a connection to the alternate world, so they either found another gate or  found something else to power the monsters. 

What new Upside Down creatures are unveiled in Season 4?

In Season 4, we’re introduced to the Demobats, winged creatures that travel in swarms. Hellfire Club leader Eddie Munson (Joseph Quinn) distracts them to save Dustin and heroically dies from their vicious bites. 

Plus, a new monster is targeting the teens of Hawkins, mutilating their bodies and panicking the townsfolk. Dustin and the rest of the crew later uncover that Vecna (Jamie Campbell Bower) is behind the murders in an elaborate plan to open four Upside Down gates across town. 

When Vecna briefly kills Max, who ends the season in a coma, it opens massive rifts all over Hawkins — which local officials deem an earthquake. 

In Russia, Hopper, Joyce, and Murray kill off the pet Demogorgons. The creatures’ connection to the hive mind is weakened by Steve (Joe Keery), Robin (Maya Hawke), and Nancy’s (Natalia Dyer) attack on Vecna at the Creel House, giving the adults the upper hand in their fight. 

What does Season 5 reveal about the Upside Down?

Following the opening of the rifts at the end of Season 4, the military has placed Hawkins under quarantine. Secretly, though, officials are conducting experiments on various Upside Down creatures in their shadow-dimension military base. 

In Episode 5, Dustin discovers from Dr. Brenner’s (Matthew Modine) journals that the Upside Down is not another world like he thought. “It’s a wormhole,” he tells Steve — a “bridge between two points in time and space” connecting Hawkins to yet another world. 

“We love the film Bridge on the River Kwai,” Matt Duffer says of the 1957 war film directed by David Lean. “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.” 

Brenner’s notebooks contain a trove of additional information about the Upside Down, including the giant sphere floating above Hawkins Lab. Dustin theorizes that it’s a shield generator using dark magic to power the flesh wall, an energy shield around the Upside Down, like the Death Star II’s protective energy shield in the 1983 movie Star Wars: Return of the Jedi. But he’s wrong. 

Vecna didn't create the sphere — science did. It’s composed of exotic matter, a single source of energy, and it’s what’s keeping the highly unstable Upside Down together. When Nancy shoots it at the end of Episode 5, she sets off a sonic blast that’s absorbed by the flesh wall surrounding the Upside Down, and everything in the wall’s path is sucked into the void. Although Nancy’s gunshot “very clearly disturbed” the sphere, it wasn’t strong enough to destroy the Upside Down. 

In Episode 7, when Dustin shares his findings with the rest of the party, he draws the diagram of the Upside Down that the Duffer Brothers used to pitch director and executive producer Shawn Levy ahead of the final season.

“They drew this same diagram because they knew intuitively before they’d even written a word of a script that we as an audience would need help,” Levy notes. 

The image, a cylinder with a circle at both ends, illustrates how the Upside Down connects Hawkins to another world called the Abyss, named after the realm of pure chaos and evil in Dungeons & Dragons. The Abyss is actually the home of the Mind Flayer, the vines, the Demogorgons, and all of Vecna’s other creatures — not the Upside Down. 

Where did Eleven send Henry after their showdown in the Rainbow Room?

Eleven banished Henry to the Abyss, not the Upside Down, after eviscerating him in the Rainbow Room in a flashback in Season 4. As Dustin explains, Henry would have “remained lost” had Dr. Brenner not forced Eleven to search for him. When Eleven made remote contact with the Abyss, the Upside Down was formed. Ever since, “Henry and his monsters have been using it to cross right back into Hawkins,” Dustin says.

What is the Abyss?

Living up to its name, the Abyss isn’t exactly a tourist destination. The alternate dimension features a stormy yellow sky over a jarring terrain of steep canyons and sharp rocks jutting out at odd angles. In this unfriendly world, Holly (Nell Fisher) and the other children Vecna has kidnapped are entombed in the spires of a grotesque fixture known as the Pain Tree.

Now armed with a full understanding of the Upside Down, our heroes are gearing up to stop Vecna from merging the Abyss with Earth and to destroy all the darkness connected to him, including the Upside Down. 

While you await that final battle, stream the first seven episodes of Stranger Things 5 now. Then tune in for The Finale on New Year’s Eve at 5 p.m. PT. Find out when the last episode arrives in your part of the world here. Unpack that explosive Volume 2 ending here. And test your knowledge of Stranger Things with our superfan quiz.