And that's a common theme in the legendary composer's work — you can hear it in Interstellar and in Sherlock Holmes.

But in Dunkirk, the ticking makes way for an overwhelming orchestra that seems like it's rising higher and higher, but never actually does.

It's so tense, it makes you cling to your seat.

That is because Zimmer is taking advantage of an auditory illusion caused by something called a Shepard tone.

It consists of several tones separated by an octave, layered on top of each other.

As the tones move up the scale, the highest-pitched tone gets quieter, the middle pitch remains loud, and the lowest bass pitch starts to become audible.

Because you can always hear at least two tones rising in pitch at the same time, your brain is tricked into perceiving a constant ascending tone.

Loop it all together and it sounds like a piano scale going on for infinity.

When the transition between tones is continuous, it's called a Shepard–Risset glissando.

And it can sound really spooky.

This can happen in the opposite direction too.

You can hear it in the Endless Stairs in Super Mario 64 and in Pink Floyd.

It's like a barber's pole of sound, constantly seeming to rise without actually going anywhere.

Put that in a soundtrack and it creates this sound of rising tension that carries the screenplay forward.

Christopher Nolan loves this illusion — you can hear it in the Batpod sound effect in the last two Dark Knight films and in the music of The Prestige, composed by David Julyan.

Nolan's films are often all about time — how it warps in space, in our dreams, and in our memories — and there's tension that comes with that.

An illusion like this makes that tension palpable — all it takes is clever sound design.

I just wanted to give a shoutout to Jason Guerrasio at Business Insider for doing this great interview with Christopher Nolan.

In the interview, Nolan confirms that a lot of the soundtrack was built around the effect of a Shepard tone.

And he also reveals that the ticking that Hans Zimmer uses at the very beginning of Dunkirk is actually a recording of a watch that Christopher Nolan owns.