Far Out Corners, by Bob Miller, 1978. Photo by Susan Schwartzenberg
Far Out Corners is an extension of an illusion called the Necker Cube, an outline which can be interpreted in two equally feasible ways–concave or convex. The actual corners here are concave, but the way they are lit encourages the eye to see them as solid cubes.
Here’s how to build one at home: http://www.exploratorium.edu/snacks/far_out_corners/