"Dunkirk." Warner Bros.

"Dunkirk" didn't just mark the first time Christopher Nolan had made a war movie; it was also one of the rare times a filmmaker had 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 troops 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 perform a landing in the English Channel.

An Imax camera was strapped into the cockpit, filming as actor Jack Lowden struggled to get out. Viewers watch as the water begins to fill the cockpit, delivering one of the movie's most dramatic scenes.

During filming, however, the plane with the camera still inside sank 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 of the kind used on the movie cost about $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 are some shots of the dramatic scene 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 provides more flexibility in the kinds of shots you can do, Nolan's footage from inside the cockpit 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: