We’ve seen 20 (21 if you count the unofficial Never Say Never Again) James Bond films since the release of Thunderball 50 years ago, but it still hovers over Bond fandom like few other entries in the franchise can. It’s not necessarily the best movie in the ongoing spy saga (a lot of fans would give that honor to Goldfinger), nor is it the most expensive (that’s the recently released Spectre), but it is one of the most iconic Bond installments, a film that came along at exactly the right time and courted fans in exactly the right way. So, to celebrate its enduring legacy five decades after its release, here are 10 fascinating facts about the fourth big-screen James Bond adventure, from its odd origins to its wild stunt work.

1. IT STARTED AS A FILM, THEN BECAME A BOOK, THEN BECAME A FILM AGAIN.

In 1959, Bond creator Ian Fleming began considering a film version of his character, and collaborated with producer Kevin McClory and writer Jack Whittingham on a screenplay treatment. Fleming eventually tired of the movie business, and went back home to Jamaica to write his next Bond novel, Thunderball. McClory later sued, claiming the novel used elements from the film they’d worked on together. The suit settled out of court, but McClory was granted certain rights to Thunderball in the process, and ultimately served as a producer on the movie. Nearly two decades after Thunderball was released, he served as an executive producer on Never Say Never Again, a Bond film that saw the return of Sean Connery in the title role for the first time in more than a decade (produced by Warner Bros. and not Bond’s home studio of MGM). The plot is in many ways identical to Thunderball.

2. TERENCE YOUNG WAS NOT THE ORIGINAL DIRECTOR.

Producers Albert R. “Cubby” Broccoli and Harry Saltzman originally wanted Guy Hamilton—fresh off the success of the third Bond film, Goldfinger—to direct Thunderball, but Hamilton didn’t feel he had the energy.

“I was drained of ideas,” Hamilton said in an interview for The Making of Thunderball. “I was very fond of Bond, but felt that I had nothing to contribute until I’d recharged batteries.”

Producers then turned to Terence Young, who’d directed both Dr. No and From Russia with LoveThunderball would be his final Bond film. Hamilton later went on to direct Connery again in Diamonds Are Forever, and Roger Moore in Live and Let Die and The Man with the Golden Gun.

3. HUNDREDS OF ACTRESSES WERE CONSIDERED FOR THE ROLE OF DOMINO.

The role of Domino Derval was set to be the most complex Bond Girl yet, and producers looked at hundreds of actresses for the role. Among the biggest names were Julie Christie, Raquel Welch, and Faye Dunaway, but the role eventually went to Claudine Auger, a former Miss France. Even Thunderball co-star Luciana Paluzzi auditioned for the role, but was actually overjoyed when she found out she’d be playing villain Fiona Volpe instead.

4. THE JETPACK REALLY WORKED, BUT THE PILOT WOULDN’T FLY WITHOUT A HELMET. 

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The Bell Rocket Belt used in the film’s opening sequence was a real working jetpack, and two qualified pilots were flown to France to operate it for the moment when Bond lifts off. Bill Suitor, who flew the jetpack on camera, was initially asked if he would mind flying without a helmet so that Bond could look cooler. Suitor refused for safety reasons, which is why Connery wore a helmet in the final film.

5. A CHASE SCENE NEARLY KILLED A STUNTMAN.

For the scene in which Fiona Volpe uses rockets launched from a motorcycle to blow up Count Lippe’s car, stuntman Bob Simmons was tasked with driving the car, then leaping out after the explosion took place. Simmons leapt out as the car crashed into a ditch, then seemed to disappear. As the crew frantically searched for him, Simmons walked up behind director Terence Young and asked him if he’d done the scene right. Footage from another angle later showed that Simmons had actually tried to stand up in the ditch and fallen backwards into the flaming car before escaping the scene.

6. THE BRITISH MILITARY THOUGHT BOND’S MINIATURE OXYGEN TANK WAS REAL. 

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Early in the film, Q gives Bond a tiny breathing apparatus that allows him to survive underwater for several minutes, and Bond puts it to good use when trapped in a closed pool with a bunch of sharks. The scene was so convincing that a member of the Royal Engineers called chief draftsman Peter Lamont and asked him how long the apparatus actually worked. Lamont replied “as long as you can hold your breath.” When the engineer countered that Bond was underwater for several minutes onscreen, Lamont replied it was “the skill of the editor.” The engineer eventually hung up.

7. THREE PEOPLE NARROWLY ESCAPED SHARK ATTACKS.

The scenes involving villain Emilio Largo’s shark-filled pool proved difficult for a number of reasons. Stuntman Bill Cummings, for a shot in which Largo throws someone into the pool to be devoured, asked for 250 pounds of hazard pay because he was actually being asked to leap onto a live shark (he survived). For the scenes in which Bond himself was trapped in the pool, things got trickier. Sean Connery was wary of swimming unprotected with live sharks, so production designer Ken Adam constructed an underwater partition made of Plexiglas, with one problem:

“What I didn’t tell Sean was that I could only get so much Plexiglas,” Adam later said.

So, there was a four-foot gap in the partition, and sure enough, one of the sharks managed to find it, leaving Connery with just enough time to escape the pool.

For a shot in which a shark swims toward Bond as he’s exiting the pool, missing him by inches, the crew decided to use a dead shark pulled by wires to reduce the danger. Special effects coordinator John Stears got in the pool to control the shark, surrounded by other live sharks, and as they began to shoot it became clear the shark wasn’t really dead. As it began moving, other sharks took notice, and a feeding frenzy ensued, leaving Stears in the middle of a bloodbath. As Stears yelled “Get me out of here!” Young shouted “Turn the cameras!,” hoping to capture the scene on film. Stears survived, and went on to win an Oscar for Best Visual Effects for Thunderball (he won a second Oscar in 1978 for a little film called Star Wars).

8. THE FILM’S CLIMACTIC EXPLOSION WAS BIGGER THAN ANYONE EXPECTED.

For the scene in which Largo’s yacht, the Disco Volante, crashes onto rocks and explodes, Stears turned to U.S. Air Force Lieutenant Colonel Charles Russhon, who’d also helped the Bond crew gain access to Fort Knox for Goldfinger. Russhon provided the production with experimental rocket fuel to help with the explosion, and Stears­—having no idea how powerful the stuff really was—loaded the yacht with it. The resulting explosion was so huge that it launched the boat into the air, almost causing it to land on the crew.

“I said ‘I don’t want to worry you guys, but the boat is coming down on top of us,’” Stears later recalled.

When the crew returned to Nassau after shooting the scene, they discovered the power of the explosion had also shattered windows all along Bay Street. They were 30 miles away from Nassau when the explosion occurred.

9. THE ORIGINAL THEME SONG WAS VERY DIFFERENT.

For the film’s theme, composers John Barry and Leslie Bricusse initially wrote a tune called “Mr. Kiss Kiss, Bang Bang” (which is why there’s a location called the “Kiss Kiss Club” in the film), and asked “Goldfinger” vocalist Shirley Bassey to sing it. Dionne Warwick later re-recorded the song, and producers were initially going to include it in the film, as long as the lyrics began after the title Thunderball appeared onscreen. Eventually, though, fearing there would be confusion if the theme song didn’t include the movie’s title in the lyrics, the song was pulled. Barry and lyricist Don Black then wrote “Thunderball,” performed by Tom Jones in the final film.

10. SEAN CONNERY DID NOT GO TO ANY PREMIERES BECAUSE FANS WERE TOO ENTHUSIASTIC.

Thunderball was a massive success, and the film’s release was met with an incredible reception. Some theaters stayed open 24 hours a day to keep screening the movie, and cast members attended various premiere events around the world (Molly Peters, who played Patricia Fearing, believes she saw the movie 16 times that year). One actor who didn’t go to the premieres, though: Sean Connery. Why? Because the Goldfinger premiere in Paris had been so huge that a female fan climbed into the car he was driving, prompting him to shy away from the Thunderball attention.

Additional Source: The Making of Thunderball and The Thunderball Phenomenon (1995)