You need ***red/cyan or red/green glasses*** in order to view this image correctly.
This is an anaglyph, a special kind of a stereo photo. When viewed with the naked eye, it looks as if a cheap lens with a lot of chromatic aberration would have been used.
An anaglyph is a combination of two separate shots, taken at a horizontal distance of about 6 cm. The red channel of the one and the cyan channel of the other shot are combined for the resulting image. If you would look at this image with the right colored glasses you certainly would consider it an example for an illusion; I am afraid that not everybody may have this kind of glasses handy right now. See Jason's comment below for a workaround.
A stereo pair version can be found here:
http://www.pbase.com/gerald_h/image/57440655
(No glasses needed, but you will have to be able to squint.)
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