[Editor's Note: Spoilers for Game of Thrones Season 7 Episode 6, "Beyond the Wall" below!]

"No matter how great your visual effects or art department is," said Game of Thrones Showrunner David Benioff, "there's nothing quite like the real thing, if you can find it. And this place exists. It's real. It's cold as hell."

That place—the location that served as the backdrop of Season 7, Episode 6, "Beyond the Wall"—is a remote glacier region of Iceland.

"It doesn't even look real," said Showrunner D.B. Weiss. "I worry that people will think the blues of those glaciers are some kind of cheap special effect. It doesn't look like anywhere else. But the more spectacular-looking it is, the more difficult it is to shoot there."

In a new behind-the-scenes video for the episode, a producer revealed that the Icelandic technical crew was tasked with transporting and setting up all of the equipment necessary for the shoot to the remote location in total darkness. This was the only way for production to maximize the small window of light in the Northern reaches of the globe. 

"There's an extent to which nothing ever surprises you on this job anymore," said Joe Dempsie, who plays Gentry, "but this—you can't help but be impressed. It's nuts."

Even by Game of Thrones standards, the Frozen Lake sequence was as complex as they come. Says Weiss: "This sequence had to be so well-conceived on a visual level by [Alan Taylor, director of Episode 6] and his team, with hundreds of people and weather and VFX and fire, and still have that level of artistry about it. It takes a different level of directing."

Taylor himself said that his overarching goal was to "show them defending a perimeter, and as you lose, it falls back further and further." For him, the challenge was maintaining order amidst pure chaos. "You're herding kittens," he said. "You can't see anything except people whacking each other."

Credit: HBO

When Weiss and Benioff informed the crew of their intentions to create a zombie polar bear for the episode, "the VFX guys told us in the nicest possible way, 'Fuck you.'" Benioff recalls. "But we put our four feet down and said, Goddammit! We want a zombie polar bear."

To create the undead bear, the VFX team did a crude visual mock-up. Then, they placed the actors on wires in a green-screen studio, thrashing their bodies around to simulate the violent movements associated with fighting a bear. In shots where an actor had to come face-to-face with the bear, the VFX team built a flaming sword in a wire cage that stood in for the bear's jaws.

Building the Frozen Lake (and Dragons)

Because it would have been irresponsible to endanger cast and crew by shooting atop a real frozen lake, the Game of Thrones crew had to create one from scratch. They did so in Wolfhill Quarry in Belfast, where they painted the cliffs and "snowed" them with 3.5 thousand bags of fake snow and six machines. Every day, they had to scrape the cliffs off and begin again.

The lake was built with cement and designed to make the ice look three-dimensional and transparent. Production designers had to match the Icelandic location as best they could. "It was like we were building an airport," said a designer. "The scale of it was enormous."

To make the dragon feel like an authentic element of the scene, the VFX team modeled a section of its back and laser-cut a full-scale piece of polystyrene to place on the frozen lake.

All told, "Beyond the Wall" was a massive technical achievement for the Game of Thrones cast and crew.

Here are more behind-the-scenes secrets from Game of Thrones.