The staircase of sound that never ends

In 1964, psychologist Roger Shepard of Bell Labs described a baffling auditory illusion: a scale that seems to rise endlessly yet never gets anywhere. It's the sonic equivalent of Escher's impossible staircases.

The trick is to stack the same note across several octaves at once. As the notes climb, the highest octaves gradually fade out while new low ones fade in — so smoothly that the brain never notices the handover. The result: tension that builds and builds and never resolves.

Hans Zimmer turned it into a secret weapon. The score of Dunkirk is built on Shepard tones, which is why the film keeps your stomach in knots until the very end. It also plays on the endless staircase of Super Mario 64. Next time a scene stresses you out and you can't say why, the culprit may not be the picture — it may be a staircase that never ends.