Dunkirk’s “mole” isn’t a spy. But if you missed that detail, you’re not alone.

Christopher Nolan’s film Dunkirk runs along three parallel time tracks, and early in the film, those tracks are indicated by supertitles on the screen. One of those supertitles refers to “the mole.”

If you’re American but not much of a World War II or architectural buff, you may — like me — have assumed that “the mole” referred to one of the characters in the film. Besides the furry animal burrowing underground, most Americans think of a mole in terms of its second definition: “a spy who achieves over a long period an important position within the security defenses of a country.”

That definition seems to fit just fine in the context of Dunkirk, which takes place during WWII. But it turns out that’s not what the film intended with its use of the term “mole.”

The evacuation of British and French troops at Dunkirk was largely accomplished through the use of two long concrete jetties that protected the beach. The name of those structures? Moles. (The word has roots in middle French and Latin.)

During the Battle of Dunkirk, the harbor was rendered unusable by German bombing, and the big boats couldn’t get to the soldiers on the beach, which meant evacuation initially progressed very slowly. But then the Royal Navy Captain W.G. Tennant came up with the idea of evacuating the soldiers off the East Mole, which had deep water on each side of it.

And it worked brilliantly. In the end, only about a third of the soldiers were evacuated from the beach itself; about 200,000 were evacuated off the East Mole. If you visit Dunkirk today, you can walk on a concrete section of the mole, though the wooden part was lost in a storm in the 1970s.

This plan actually plays out in Dunkirk, and the commanding officers talk about it; one of the reasons they’re so desperate to move a sinking ship near the beginning of the film is that if it sank near the mole, big ships wouldn’t be able to get near it. But part of Dunkirk’s immersive experience means it’s actually a bit hard to make out what they’re discussing, especially if you don’t know military lingo — and especially if you’re an American also trying to decipher various British accents. (For many Americans, Dunkirk is unusual for being a WWII blockbuster with no American characters.)

The term isn’t used much in the United States, so many of the film’s American viewers — or at least the ones who aren’t up on their WWII history — might not pick up on its meaning. But Nolan himself was more likely to be familiar with it even before the film: He was born in London to an American mother and an English father, and the family split their time between London and Evanston, Illinois. He has dual citizenship.

Meanwhile, one of the characters in that section of the film is under suspicion of being a spy. So if you (like me) didn’t realize what the term meant, you probably weren’t alone. It’s another remarkable part of the story of Dunkirk.

Thank you to the astute reader who emailed me to point out what the real “mole” is in Dunkirk.