Christopher Nolan used an old Hollywood trick to salvage footage that sank in the water while shooting 'Dunkirk'

"Dunkirk" didn't just mark the first time Christopher Nolan has made a war movie, it was also one of the rare times a filmmaker has ever shot a majority of a movie with an IMAX camera.

So, Nolan did a lot of things he didn't know were possible until he actually did them.

And in one instance, a blunder on set led to a fascinating discovery.

In exploring the historic evacuation of Allied forces from Dunkirk, France during World War II, the movie highlights British pilots fighting German planes to protect the Allied soldiers on the ground. The dogfight sequences in the movie are thrilling, and in some cases very authentic. The production stayed away from CGI as much as possible, and in one sequence Nolan had a replica Spitfire plane do a water landing in the English Channel.

Inside the plane, an IMAX camera was strapped into the cockpit, as actor Jack Lowden struggled to get out. In the scene, we watch as the water begins to fill the cockpit, delivering one of the movie's most dramatic scenes.

When filming the scene, however, the plane with the camera still inside sank into the water quicker than anyone on the crew thought possible. Nolan was certain the footage had been lost.

"In the hours it took to retrieve the IMAX camera, its housing, which was a big plastic barrel, actually had a hole in it and the entire thing filled with water," Nolan told Business Insider.

IMAX told Business Insider an IMAX camera used on the movie costs around $1 million.

"But we called the lab and they clued us into an old-fashioned technique that used to be used on film shoots," Nolan said. "You keep the film wet, you unload the camera, and you keep it damp the whole time. We shipped it back to Los Angeles from the set in France, and they processed it before drying it out and the shot came out absolutely perfect and it's in the film." 

Here's some shots of the dramatic scene that Nolan thought would never see the light of day:

"Try doing that with a digital camera!" Nolan said with glee. In the age of digital, the director is one of the last to be a major supporter of shooting on film. Though shooting digitally is cheaper and gives you more flexibility in the kinds of shots you can do, Nolan's footage really would have been destroyed if "Dunkirk" weren't shot on film.

"Dunkirk" opens in theaters on Friday.

Catch a glimpse of the Spitfire water landing at the end of the TV spot for the movie below: