A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition of sine waves separated by octaves. When played with the base pitch of the tone moving upwards or downwards, it is referred to as the Shepard scale. This creates the auditory illusion of a tone that continually ascends or descends in pitch, yet which ultimately seems to get no higher or lower.Listen to it. It's like the sound of a plane coming in low for a landing, but never actually passing overhead. It'll drive you batty.
(Via Defective Yeti, which accurately describes it as "a color that shouldn't exist or an angle that doesn't add up.")