In many natural scenes, portions of occluding surfaces and contours can be camouflaged by having an identical color as their background. It is now well known that the visual system will often generate illusory contours and surfaces in order to fill-in this missing information. It is widely believed that the visual system does this in order to provide the best overall “explanation” of the images that form in our eyes. In this illusion, four circular disks are simply translated back and fourth over a thin square outline. Although all of the image data is fully “explained” by the visible disks, the visual system generates a strong and clearly visible additional illusory contour inside the square outline. This illusion cannot be understood with any known mechanism or theory that has been proposed to explain the formation of illusory contours. This quicktime movie is best viewed by downloading it and playing it in “loop” mode.