Legendary Pictures took a gamble on Pacific Rim, Guillermo del Toro’s 2013 monster/robot slugfest. Since it wasn’t based on a preexisting franchise, it lacked a built-in fanbase. That can be a serious drawback in our current age of blockbuster remakes and reboots. The movie underperformed domestically; in America, it grossed just over $100 million against its $180 million budget. Yet Pacific Rim was a huge hit overseas and acquired enough fans to earn itself a sequel, Pacific Rim Uprising, which arrives in theaters this week. Here are 10 things you might not have known about the movie that started it all.

1. THE PREMISE CAME TO SCREENWRITER TRAVIS BEACHAM WHILE HE WAS TAKING A STROLL.

Warner Bros.

One foggy day in 2007, Beacham—who’d recently moved to California—was walking along Santa Monica Beach. As he looked out at the Ferris wheel on the city’s eponymous pier, he pictured a looming sea monster. Then he imagined an equally large robot gearing up to fight the beast. “They just sort of materialized out of the fog, these vast godlike things,” Beacham said. He decided to pursue the concept further after coming up with the idea of human co-pilots who’d need to operate their robot as a team, which added a new thematic dimension.

“I didn’t know I had something I wanted to write until I realized these robots are driven by two pilots, and what happens when one of those people dies? What happens to the leftovers? Then it became a story about loss, moving on after loss, and dealing with survivor’s guilt," Beacham said. "That made the monsters scarier because now you care about the people who are in these robots.”

2. CONCEPT ARTISTS PUT TOGETHER “ABOUT A HUNDRED” DIFFERENT KAIJU AND JAEGER DESIGNS.

Pacific Rim was picked up by Legendary Pictures and handed over to director Guillermo del Toro. A huge fan of monster cinema, del Toro enthusiastically co-wrote the final screenplay with Beacham. Sixteen concept artists were hired to sketch original robot and creature designs for the film. “We would get together every day like kids and draw all day,” del Toro told the New York Daily News. “We designed about a hundred Kaijus and about a hundred Jaegers and every week we would do an American Idol and we would vote [some of] them out.”

3. CHARLIE DAY WAS CAST AFTER DEL TORO CAUGHT HIS PERFORMANCE IN AN EPISODE OF IT’S ALWAYS SUNNY IN PHILADELPHIA.

In “Charlie Kelly: King of the Rats,” the tenth episode of It's Always Sunny in Philadelphia's sixth season, Charlie Day’s character gives us a darkly comedic monologue about rodent extermination. Little did the actor know that the performance would open a big opportunity for him. Impressed by the rat speech, del Toro offered Day the part of Dr. Newton Geizler, Pacific Rim’s socially-inept kaiju expert. “He said to himself, ‘That’s my guy. That guy should be in my next movie because if he killed rats, he can kill the monster,’” Day recalled during an appearance on Late Night with Jimmy Fallon. On the movie set, del Toro often joked about how much he enjoys It’s Always Sunny. As a way of repaying his director, Day helped get del Toro a minor role in the series.

4. THEY BUILT A FOUR-STORY JAEGER HEAD.

Most of the film’s special effects were computer-generated, but not everything was digital. For the robot cockpit scenes, del Toro had his team build the interior of a full-scale Jaeger head. The finished product stood four stories tall and weighed 20 tons. And like a Tilt-A-Whirl from hell, it was designed to rock around violently on its platform via a network of hydraulics. Once inside, the actors were forced to don 40-pound suits of armor. Then the crew strapped their feet into an apparatus that Charlie Hunnam has compared to a high-resistance elliptical machine.

Certain shots also required del Toro to dump gallons of water all over his exhausted, physically-strained stars. So yeah, the experience wasn’t much fun. “We saw every one of the actors break down on that set except for the female lead actress Rinko Kikuchi," del Toro said. "She’s the only actor that didn’t snap."

5. GIPSY DANGER WALKS LIKE JOHN WAYNE.

Del Toro wanted Gipsy Danger, his ‘bot, to have the self-confident air of a wild west gunslinger. To that end, he and concept artist Oscar Chichoni developed a swaggering gait that was based on John Wayne’s signature hip movements. The Jaeger’s Art Deco-like design was influenced by the Chrysler and Empire State Buildings.

6. RAGE AGAINST THE MACHINE'S TOM MORELLO WORKED ON THE SOUNDTRACK.

Hailed as the “fortieth greatest guitarist of all time” by Rolling Stone, Rage Against the Machine's Tom Morello rocked the MTV generation with hits like “Bulls on Parade” and “Killing in the Name.” Pacific Rim bears his mark as well. The film’s lead composer was Ramin Djawadi, whose other works include the Game of Thrones theme. Wanting to add a “rock element” to the Pacific Rim soundtrack, he and del Toro reached out to Morello. The guitarist didn’t need much persuading.

“When they asked me to put some giant robot riffs and screaming underwater monster licks on the film score, I was all in,” Morello said. Djwadi was pleased with the rocker's contributions to the project. As he told the press: “Tom’s unique style and sounds really defined our robots.”

7. ONE JOKE IN THE HONG KONG SCENE CALLED FOR A MINIATURE OFFICE SET.

A definite highlight of this movie is Gipsy Danger’s duel with the winged kaiju Otachi in downtown Hong Kong. Both characters were computer-generated, as were the majority of the streets, cars, and towers in this epic sequence. However, there is one moment which was at least partly realized with practical effects. Gipsy punches through the wall of an office building early in the fight. We see her fist rip through a series of cubicles and gradually decelerate until it lightly taps a chair with just enough force to set off a Newton’s Cradle desktop toy. For that shot, effects artists at 32Ten Studios constructed a miniature office building interior featuring 1/4-scale desks, cubicles, and padded chairs. The level of detail here was amazing: 32Ten’s staff adorned each individual workspace with lamps, computers, wastebaskets, and teeny, tiny Post-it notes.

8. THREE DIFFERENT ENDINGS WERE SHOT. 

Kerry Hayes, Warner Bros. Entertainment Inc. and Legendary Pictures

Audiences reacted strongly to Kikuchi’s character Mako Mori, who inspired an alternative to the famous Bechdel test. Some critics praised the culmination of her relationship with Raleigh Beckett (Hunnam). Although it’s common practice for the male and female leads in an action flick to end their movie with a smooch, Mori and Beckett share a platonic hug as Pacific Rim draws to a close. Del Toro revealed that he shot three different versions of that final scene. “We did one version where they kiss and it almost felt weird. They’re good friends, they’re pals, good colleagues,” del Toro said.

9. PACIFIC RIM WAS DEDICATED TO RAY HARRYHAUSEN AND ISHIRO HONDA.

At the end of the credits, there’s a tribute that reads: “This film is dedicated to the memories of monster masters Ray Harryhausen and Ishiro Honda.” Harryhausen passed away on May 7, 2013—two months before Pacific Rim’s release. A great stop-motion animator, he breathed life into such creatures as the towering Rhedosaurus in 1953’s The Beast From 20,000 Fathoms.

Ishiro Honda was another giant of the kaiju genre, having directed Rodan, War of the Gargantuas, and numerous Godzilla films. Del Toro has great respect for both men. When Harryhausen died, the director said, “I lost a member of my family today, a man who was as present in my childhood as any of my relatives.” He also adores the Japanese monster classics and says he’d love to see a Pacific Rim-Godzilla crossover someday. Maybe it’ll happen.

10. GUILLERMO DEL TORO LIKED ONE FAN-MADE TRAILER SO MUCH THAT HE INVITED ITS CREATORS TO THE MOVIE’S PREMIERE.

If you’re not familiar with the practice of “Sweding,” let us fill you in: The 2008 comedy Be Kind, Rewind is about two co-workers at a VHS rental store who accidentally erase every tape in stock. Hoping to save their skins, they create ultra low-budget remakes of all the films they’ve destroyed using cardboard sets and cheap costumes. It’s a process these guys call “Sweding” as a ploy to convince everyone that their (unintentionally hilarious) knockoffs were produced in Sweden. Since Be Kind, Rewind was released, Sweding has become a legitimate art form.

When Pacific Rim’s first trailer debuted in 2013, YouTubers Brian Harley and Brodie Mash created a shot-for-shot, Sweded duplicate of the preview. Instead of state-of-the-art CG effects, their version used toy helicopters, duct-tape monster masks, and an ocean of packing peanuts—and del Toro loved it. At WonderCon 2013, he praised the video, saying that it inspired the editing used in Pacific Rim’s third trailer. Harley and Mash happened to be at the same gathering. When del Toro met the comedic duo, he exclaimed “I loved it! My daughters loved it, we watched it a bunch of times!” Then he invited the Sweding duo to attend Pacific Rim’s premiere in Hollywood.