Shepard-Risset Glissando (click to play!)
Risset Rhythm (click to play!)
A descending Shepard-Risset Glissando seems to get lower but at the same time seems to remain constant in pitch. Since no sound can change in pitch and at the same time remain constant in pitch, you experience an apparently impossible sound. (Compare a color that—impossibly—looks both red and green.)
Similarly, an accelerating Risset Rhythm (based on similar principles) seems to speed up but at the same time seems to have a constant tempo. It therefore constitutes an apparently impossible rhythm.
To maximize your auditory bewilderment, play both tracks simultaneously.
Here is the question that interests me (it has received almost no attention from philosophers): Why do we find these sorts of experiences engaging? (I hesitate to say that we find them pleasurable. Some people do not—just as some people don't like magic, or philosophy!) This is one of the motivating questions of my project, Antinomic Aesthetics.
For a musical composition that incorporates the Shepard-Risset Glissando, see Jean-Claude Rissett's "Computer Suite from Little Boy" (1968).
Finally, for more information about Shepard Tones and the Shepard-Risset Glissando, see this Wikipedia page.