This is an example of an anamorphic illusion created by Julian Beever. This sidewalk drawing was drawn in a special distortion in order to create an impression of 3 dimensions when seen from one particular viewpoint This guy got the skills...
For those who can't view it, the URL is http://eyetricks.com/1701.jpg You might have to try to enter ot from eyetricks.com's homepage so it catches a referrer. There is a gallery of this guys work somewhere. I've seen it, and it's pretty amazing.