How could we have missed it? Hundreds, perhaps thousands, of visual scientists, psychologists, neuroscientists, visual artists, architects, engineers and biologists all missed it—until 2007. The “it” in question is the leaning tower illusion, discovered by Frederick Kingdom, Ali Yoonessi and Elena Gheorghiu, all then at McGill University. In this illusion, two identical side-by-side images of the same tilted and receding object appear to be leaning at two different angles. This incredible effect was first noticed in images of the famed Leaning Tower of Pisa, but it also works with paired images of other receding objects.