The "sonic barberpole" illusion invented by psychologist Roger Shepard at Bell Labs. The illusion consists of a seemingly endlessly rising or falling set of tones. The trick is done by simultaneously sweeping eight (or so) pure tones (i.e., sine waves) tuned exactly one octave apart. The human ear/brain has a really hard time figuring out which pure tone is the fundamental, so it "slips" periodically, just like an eye watching a barberpole (or looking at an Escher staircase). To conceal the tones' appearing and disappearing they fade in at one end of the cycle and fade out at the other.

The Science of Musical Sound--John R. Pierce, Scientific American Library
"Lab Notes: Shepard Functions"--John S. Simonton, Jr., Polyphony, 2/83

Shepard Tones.mp3 (~500k)

More examples: http://www.musicsynthesizer.com/Circuitry/shepard.html