The fourth iteration of a fractal called the Menger sponge. It’s constructed by dividing each face of a cube into squares, then removing the cube in the middle of each face, as well as the cube in the center. The process is then repeated for each of the smaller cubes, and so on. In the limit, you get the Menger sponge. It exhibits a curious combination of properties: it has an infinite surface area, yet encloses zero volume.