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Magic truly comes alive for Once Upon a Time‘s 100th episode. Smoke machines blanket an entire icy warehouse, forcing cast and crew alike to don gas masks. Dry-ice bubbles rise from the depths of an 80-foot-long pool, over which a wooden barge skims the dark surface, guided only by dim lanterns. It’s eerily quiet. That is, until someone starts synergistically singing “It’s a Small World.”

The crew has jumped aboard the ferryman’s boat, which will eventually take heroes and villains alike to the mouth of the Underworld. “You must be this tall to ride this ride,” jokes one crewman, who will spend the day retracting the ship in order to send it on the brief but slow journey into depths of Hell. Unable to see five feet in front of them, the weapons-clad cast boards the ship, ready to stuff their respirators away once cameras start rolling. Emma (Jennifer Morrison) is back in her hero gear, the requisite red leather jacket cutting through the haze, as she lies prone in the boat, waking up with a start upon their arrival.

“All we know so far is there’s a lot of smoke,” Morrison says between takes. “Visually, what I understand is that there’s going to be a more film-noir feeling and something darker, with more contrast in what you’re seeing.”

However, this is but the entrance to the Underworld. As an actual location, the Underworld looks very familiar: it’s a twisted version of Storybrooke, where its denizens aimlessly wander the town. “It’s almost identical to Storybrooke,” Lana Parrilla says. “It reminds me of when Marty McFly goes back to his old neighborhood; it’s just different and torn down, and there’s graffiti everywhere. Storybrooke doesn’t look the same at all.”

“It’s Storybrooke in the apocalypse,” Josh Dallas adds. “The place is [filled with] burned-out cars, the clock tower is smashed on the ground, and there are zombies walking around everywhere. It’s hell. It’s absolute hell.”

In a bid to put their OUAT twist on it, the producers aimed not to make it look like the fire and brimstone we’ve come to expect of Hell. “It couldn’t just look like regular Storybrooke, everything had to be slightly off,” says executive producer Steve Pearlman, who explains they mulled different iterations of the Underworld before bringing it to life. Among the early ideas they considered: Having a boiling red sky (the price tag was a little too steep), only filming at night (nixed for the sake of cast and crew) and literally flipping everything on its head (they went for slightly off-kilter instead of full on topsy-turvy).

“We’ve been going through literally each of our regular sets — from Granny’s dinner to Mary Margaret’s apartment to the library — and figuring out what is the Underbrooke tweak on the set,” Pearlman says, hinting that there’s a specific reason why Underbrooke looks so familiar, which will be revealed when the show returns. “For everything, it’s been a little different. The concept in the diner is that there are other patrons there, but we’re playing with the idea that everyone that’s in the diner is there not for any kind of social reason, not because they enjoy the food, it’s just what they do. They have to eat, and they’re eating for sustenance only. It’s little touches like that that our loyal viewers will notice that are slightly off. In Regina’s office, we thinned out a lot of the knickknacks and we put urns, noticeably dead flowers where they would normally be live flowers, we put black apples instead of red apples. In Mr. Gold’s shop, we’ve taken out all of the display counters and we’ve put in the pedestals like we have in his castle. Each pedestal has one item on it. We have the spinning wheel there.

“Everything is lit with a red hue also,” he continues. “It’s definitely noticeable. It’s all natural, believable. We’re not putting fire everywhere. Our design outside of Storybrooke is not that there’s fire everywhere, but smoke stacks. It looks normal for an urban city, it’s very unusual for Storybrooke. What we will eventually find out by the end of the 100th episode is that underneath Storybrooke is the Underworld where Hades is living. He’s in this cavern where the steam is emanating from.”

Unlike the Storybrooke above, however, this one is populated with characters of the past who have unfinished business. Among them, Cora (Barbara Hershey), Peter Pan (Robbie Kay), Milah (Rachel Shelley), and Cruella de Vil (Victoria Smurfit), enabling “us to relive some of the original Once Upon a Time season 1 themes,” Ginnifer Goodwin says.

“It turns out that we’re calling it Hell and the Underworld, but it’s a place of purgatory and a place of lost souls who are damned,” Dallas says, teasing that David and Mary-Margaret will come across a very familiar face (find out who here). “It’s the weigh station on their way to hell. Once they get there, they’re going to discover that possibly some of these people can be saved and brought out of there, because maybe they’re misplaced in this world.”

But also maybe some of those people don’t want their help, and instead look to realize what they couldn’t do in the land of the living. “We are facing people that have unresolved business with us — mostly with Regina or Gold, but Emma did kill Cruella,” Morrison says. “Snow has done her share of dark things in the name of good in the past as well, and so has David. We’ve all done things in the past that would put people in limbo and have unfinished business because of us. We start to realize there may be more of a mission in Hades than just getting Hook back.”

“It’s a scary place for Regina, because she’s killed so many,” Parrilla says. “We know that she’s going to be confronted with a lot of these souls, and a lot of people that she has harmed. She’s terrified. Also, having Henry [Jared Gilmore] there makes her even more vulnerable because that’s her son. If all these people are out to get her, then his life is in danger. That’s why all these souls are down there, because they have unfinished business. For a lot of them, we’re that business. And I would say, I’m probably most of that, as is Gold. We’re going to be confronted with a lot of angry souls.”

Don’t expect to see everyone OUAT has lost over the years in the Underworld, however. “There absolutely are rules and that gets outlined very quickly at the start of the second half of the season,” executive producer Adam Horowitz says, possibly indicating how Michael Raymond-James is returning as Neal.

For the time being, Emma and the gang are solely focused on finding Hook (Colin O’Donoghue), who sacrificed himself in the fall finale to rid the world of the Darkness only to have Rumplestiltskin (Robert Carlyle) once again become the Dark One. “There’s a deep resentment, obviously, because [Gold] took away what Hook’s sacrifice was meant to mean,” Morrison says. “It’s doubled her fury toward him, in a sense that he’s now betrayed them all, betrayed Killian. It’s the ultimate betrayal of this family, basically. There is a deep, deep fury there.” Still, “She needs to get to Hook,” Morrison adds. “She understands the power of the Dark One. She’s going to use him to get what she wants, because she knows how to use that power now.”

Having used that power herself as the Dark Swan will certainly affect Emma moving forward. “I do believe that she’s changed having been the Dark One,” Morrison says. “I think that there’s no way she could ever be the same knowing that she’s capable of doing some of the things that she’s now done. Yes, they were because there was a Darkness within her, but I think she’ll be humbled by that. She will be someone who is more apathetic to some of the other characters in a certain way because she will have also experienced what it was like to be driven by that kind of Darkness.”

And while Emma is less than pleased that Gold is now the Dark One, the cast is beyond excited for Carlyle to step back into that role. “It’s always good to go home,” Goodwin says playfully. Adds her real-life husband Dallas: “Maybe it’s in its rightful place. I think a world without Dark Ones would probably be good, but it wouldn’t be a balanced world. Who knows, maybe we actually need Dark Ones in order to see the light.”

“That’s what he was hired to do, and he’s just so good at it,” Parrilla concurs. “But we’re also really pissed at [Gold], because he set Hook up. It’s classic Gold, classic Rumple to pull an act like this. But we need him. We need him to navigate us through the Underworld. Him being the Dark One is almost kind of a good thing for us, because he is on our team. Even though he’s always pulling these tricks and schemes, he is on our team. Henry’s his grandson. There’s that connection. I don’t think he would do anything to hurt Henry at this point, but I think we’re all pissed at him.”

Even with the Dark One on their side, rescuing Hook won’t be so simple. “Hook’s experience of the Underworld so far is slightly different from everyone else’s,” O’Donoghue teases cryptically. “He’s not with them. He’s in a very complicated and difficult situation, which is really all that I can say about that. He’s not having a great time, to be honest. There’s a definite feeling of a prison. I don’t think Hook really knows what the entire Underworld is like. He hasn’t really seen a huge amount of this. He’s in a place where he hasn’t seen that Storybrooke is the Underworld.” As for why he’s there, O’Donoghue wasn’t yet clued in. “At this moment, I don’t know exactly his unfinished business or what it is, so as much as I would try to be cagey about it, there’s no point because I don’t actually know.”

Image zoom Jack Rowand/ABC

Making matters more complicated is the fact that Regina’s focus is split upon coming face-to-face with her mother for really the first significant amount of time since Cora’s heart was returned. “She’s super conflicted, because she loves her mother,” Parrilla says. “Where we left off with Cora and Regina, they were in a really good place. They were in cahoots, they were working together. Once Cora got her heart back, she actually saw her mother for who she really is and the love that Cora has for Regina. It’s bittersweet. She sees her mother and she’s super excited to see her, but she’s also sad, because she’s in hell. She had hoped that she were in a better place.”

But that sweet mother-daughter moment won’t last long with new villain Hades (Greg Germann) none too happy that these heroes and villains have entered his domain. “He’s as scary as anybody can be,” Dallas says of the flame-flicked dictator of darkness. “He’s a serious contender for the ultimate villain.”

While eccentrically unpredictable, this villain actually has a sense of humor. “If you were the Devil, how much fun would that be?” executive producer Edward Kitsis says. “We’ve had a lot of angry British villains and we’ve had some very wicked witches, and I think the thing about Hades is he’s a very dangerous fellow, but that doesn’t mean you can’t have fun being the devil.

“Hades is such [an] endlessly creative challenge,” Germann adds. “Anything goes. The creators bring him to life on the page with the same out-of-the-box unpredictability that they bring to the rest of the show.”

With that said, the heroes actually have a leg up this time around. “He is Hades, the minute you meet him you know he’s the villain,” Pearlman says. “For our characters, it’s going to be figuring out how to maneuver in a world where they know who the villain is from the get-go.”

Even with heroes and villains aligned, however, that doesn’t mean everyone will make it out of the Underworld alive. “There are seven of us who go down there,” Carlyle says. “I don’t know if seven will be coming back.”

“When you go to a place that’s surrounded by death and you think you can get out, maybe that’s not so easy,” adds Kitsis. “The main thing that they’re going to realize in the Underworld is the physical threats are there of course, but what’s more, the emotional threats. That is to say, think of your past coming back to haunt you. Who would you not want to see in the afterlife if you were the Evil Queen or Rumplestiltskin? What are the unfinished business that you now have a chance to repair? For us, the second half of the season is going to be very similar to the first season in that we’re going to see Storybrooke is going to be a place with people with unfinished business and we have to wonder if we can’t help them find their happy endings, even in the afterlife.”

Hitting 100 episodes is a happy ending in itself for Once Upon a Time, a show most doubted would be here this long. “If you go back and look the week before [we debuted], we were the No. 1 to be canceled, according to all the critics,” says Kitsis, who calls the 100th episode a love letter to the fans. “‘Sundays at 8 is the death slot!’ ‘This show will never make it!’ We got accused for being too hopeful. I remember people were saying, ‘It’s too earnest and hopeful. We’re in a world of dark people!’ To get here, we feel like it is because of the fans, because there are believers out there.” That belief also just helped score Once Upon a Time another season. Looks like, as per usual with OUAT, hope has once again won out.

Once Upon a Time returns Sunday at 8 p.m. ET on ABC. In the meantime, the OUAT cast shares their memories of filming the pilot here.