Stranger Things bosses originally had a much different vision for the series — and it did not include fans actually seeing the Upside-Down.

The shadowy underworld has become one of the most iconic parts of the Netflix horror show, with much of the conflict of both previous seasons taking place there. But that wasn't always the plan!

In the new book Stranger Things: Worlds Turned Upside Down, co-creator Matt Duffer reveals that he thought the Upside-Down — or the Nether, as it was first known — would work best if viewers heard about it but never saw it.

Netflix

In fact, the original concept was tipped to take place entirely in a coastal town called Montauk.

"We were never going to go into the Nether, as we originally called it," Matt Duffer says. "We were only planning to hear this other dimension over radios and walkie-talkies.

"We thought that'd be scarier. What we don't see is often scarier than what we do. And that worked for a while. But once we got five hours in, we felt the show really needed to open up, expand.

"It just became increasingly obvious: we had to go into the Upside-Down."

Netflix

That decision opened a gateway to Shadow Monsters, Demo-Dogs and the Demogorgon itself – the latter of whom is played on set by actual actor Mark Steger on stilts in a specially-augmented suit.

"All these monster movies we grew up loving, Aliens being the biggest, [the creature] was a stunt guy in a suit, or a movement artist in a suit," co-creator Ross Duffer explained. "We wanted something that, if you're a kid and you're in school and you're doodling, you could just sort of sketch this monster out.

Netflix

"I think there's a danger sometimes now – because you can do anything you want with computer graphics – that you can overcomplicate things. There's an elegant simplicity to the Xenomorph's design. I think that makes it much more iconic."

Stranger Things: Worlds Turned Upside Down is available for purchase today (October 30) — click here to get your copy.

Meanwhile, filming is currently well underway on Stranger Things 3, and Digital Spy has compiled an exhaustive list on which cast members will definitely be back for Stranger Things 3 on Netflix in 2019. Click here to have a look.

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