A scale that ascends without end. Each note sounds higher than the last. It never arrives anywhere.
What you're hearing is not real. Or rather: the rising pitch is not in the signal. The signal is ten sine tones, each exactly one octave apart, all climbing at the same rate. What makes it an illusion is the amplitude envelope: each tone's volume is set by a bell curve centered on the middle frequency range. As the stack ascends, the highest tone fades out and a new lowest tone fades in below. The spectral center of mass stays fixed while all the components keep rising.
The spectrum above shows this directly. The bars are the component tones — their horizontal position is their current frequency (log scale), their height is their current amplitude. The bell curve envelope is visible. You can watch the tones walk rightward under the envelope, wrap, and continue.
Your auditory system generates the continuous ascent. It tracks the rising components and extrapolates: higher, higher, higher. It never checks whether you arrived. The perception is produced by the listener, not retrieved from the sound.
Roger Shepard demonstrated this in 1964. The same structure appears in Escher's Ascending and Descending — monks climbing stairs that loop back to where they started — and in Penrose's impossible staircase. Visual and auditory systems making the same kind of error, or the same kind of completion, depending on how you look at it.
Use headphones for the clearest effect. The illusion is strongest at moderate speeds — drag the slider left to slow it down.