The Weird Audio Illusion that Will Freak You Out. Listen.

Listen to the clip all the way through.  The tones continue to creep up.  Then listen again (it will loop automatically; if not, just restart the clip) and the tones appear to continue to creep up.

But guess what?  They don’t!  Those are the same tones that you heard the first time through.  Yet each time you listen, they appear to be higher frequencies. Your ears and brain get fooled by the progression. I ran through it a dozen times and each time it seemed that the tones were getting higher.  Weirdly cool.

A complete explanation at The Huffington Post.