This optical illusion is based on the famous Leaning Tower of Pisa. It features an image of the tower that appears to be leaning at an impossible angle. Viewers are challenged to determine if the tower is actually leaning or if it is an illusion created by the artist. This illusion is
From Wikipedia
The leaning tower illusion is a visual illusion seen in a pair of identical images of the Leaning Tower of Pisa photographed from below. Although the images are duplicates, one has the impression that the tower on the right leans more, as if photographed from a different angle. The illusion was discovered by Frederick Kingdom, Ali Yoonessi and Elena Gheorghiu at McGill University, and won first prize in the Best Illusion of the Year Contest 2007.