Named after cognitive psychologist Roger Shepard, a Shepard scale is an audio illusion in which a scale seems to rise endlessly, without getting higher. The constituent pitches consist of several simultaneous octaves, which fade out at the top of the scale and fade in at the bottom. Taken out of the moving context, the actual octave register of a note is ambiguous to the ear. A toy piano displays similar ambiguity: as the length of the sounding rods at lowest keys is too short to produce a true bass note, its overtones are louder than its fundamental pitch. Taken out of context the lowest F can sound more like its C overtone, an octave and a fifth higher. This ambiguity is part of the charm of the toy piano, and Shepards Pi enjoys playing with that charm, with lots of scales that seem not to get higher, sonorities whose octave register is ambiguous, and moments where the meter and tempo could be heard in several different ways.
(Via tomflaherty on InstantEncore)
For more extreme scale action, listen to Greg Anderson playing Ligeti's Devil's Staircase Etude.