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Christopher Nolan’s Dunkirk is a nerve-wracking film. Three separate storylines tell the famed World War II tale — where 330,000 Allied forces, surrounded by the enemy, were evacuated from the Northern beaches of France — in a way that feels tense and visually stunning.

Built around a score by composer Hans Zimmer, Dunkirk hardly ever slows down from start to finish. And a lot of that has to do with how Zimmer uses an auditory illusion caused by Shepard tones.

Named after cognitive scientist Roger Shepard, the sound consists of several tones separated by an octave layered on top of each other. As the lowest bass tone starts to fade in, the higher treble tone fades out. When the bass completely fades in and the treble completely fades out, the sequence loops back again. Because you can always hear at least two tones rising in pitch at the same time, your brain gets tricked into thinking that the sound is constantly ascending in pitch.

It’s a creepy, anxiety-inducing sound. There’s a clever simulator tool you can use to explore how this works.

In an interview with Business Insider, Nolan revealed that the entire soundtrack was built around the sound effect to create a feeling of ever-increasing intensity that would unite the three storylines. Zimmer — who has scored most of Nolan’s films since The Dark Knight —wasn’t the first to build a soundtrack around the illusion; David Julyan did a similar treatment for Nolan on The Prestige score.

But in Dunkirk, the soundtrack stands on its own as a defining part of the film. It’s an auditory masterpiece.

Watch the video above to learn more about why the Shepard tone is crucial to the experience of Christopher Nolan’s Dunkirk.

Originally posted on VOX