Magic truly comes alive forOnce Upon a Times 100th episode.
Smoke machines blanket an entire icy warehouse, forcing cast and crew alike to dongas masks.
That is, until someone starts synergistically singing Its a Small World.

Credit: Jack Rowand/ABC
All we know so far is theres a lot of smoke, Morrison says between takes.
However, this is but the entrance to the Underworld.
Its almost identical to Storybrooke, Lana Parrilla says.

Jack Rowand/ABC
Storybrooke doesnt look the same at all.
Its Storybrooke in the apocalypse, Josh Dallas adds.
For everything, its been a little different.
They have to eat, and theyre eating for sustenance only.
Its little touches like that that our loyal viewers will notice that are slightly off.
Each pedestal has one item on it.
We have the spinning wheel there.
Everything is lit with a red hue also, he continues.
Its all natural, believable.
Were not putting fire everywhere.
Our design outside of Storybrooke is not that theres fire everywhere, but smoke stacks.
It looks normal for an urban city, its very unusual for Storybrooke.
Hes in this cavern where the steam is emanating from.
Its the weigh station on their way to hell.
We start to realize there may be more of a mission in Hades than just getting Hook back.
Its a scary place for Regina, because shes killed so many, Parrilla says.
Also, having Henry [Jared Gilmore] there makes her even more vulnerable because thats her son.
If all these people are out to get her, then his life is in danger.
Thats why all these souls are down there, because they have unfinished business.
For a lot of them, were that business.
And I would say, Im probably most of that, as is Gold.
Were going to be confronted with a lot of angry souls.
Dont expect to see everyoneOUAThas lost over the years in the Underworld, however.
Its doubled her fury toward him, in a sense that hes now betrayed them all, betrayed Killian.
Its 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.
Having used that power herself as the Dark Swan will certainly affect Emma moving forward.
I do believe that shes changed having been the Dark One, Morrison says.
Its always good to go home, Goodwin says playfully.
Adds her real-life husband Dallas: Maybe its in its rightful place.
I think a world without Dark Ones would probably be good, but it wouldnt be a balanced world.
Who knows, maybe we actually need Dark Ones so you can see the light.
Thats what he was hired to do, and hes just so good at it, Parrilla concurs.
But were also really pissed at [Gold], because he set Hook up.
Its classic Gold, classic Rumple to pull an act like this.
But we need him.
We need him to navigate us through the Underworld.
Even though hes always pulling these tricks and schemes, he is on our team.
Even with the Dark One on their side, rescuing Hook wont be so simple.
Hooks experience of the Underworld so far is slightly different from everyone elses, ODonoghue teases cryptically.
Hes not with them.
Hes in a very complicated and difficult situation, which is really all that I can say about that.
Hes not having a great time, to be honest.
Theres a definite feeling of a prison.
I dont think Hook really knows what the entire Underworld is like.
He hasnt really seen a huge amount of this.
Hes in a place where he hasnt seen that Storybrooke is the Underworld.
As for why hes there, ODonoghue wasnt yet clued in.
Shes 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.
She had hoped that she were in a better place.
Hes as scary as anybody can be, Dallas says of the flame-flicked dictator of darkness.
Hes 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.
Hades is such [an] endlessly creative challenge, Germann adds.
With that said, the heroes actually have a leg up this time around.
He is Hades, the minute you meet him you know hes the villain, Pearlman says.
There are seven of us who go down there, Carlyle says.
I dont know if seven will be coming back.
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?
If you go back and look the week before [we debuted], we were the No.
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, Its too earnest and hopeful.
Were in a world of dark people!
That belief also just helped scoreOnce Upon a Timeanother season.
Looks like, as per usual withOUAT, hope has once again won out.
Once Upon a Timereturns Sunday at 8 p.m.
In the meantime, theOUATcast shares their memories of filming the pilothere.