Menger
Some views of a Menger sponge (or rather it's 4th iteration). The visible edges are drawn in black, while the once occluded ones are drawn thinner, allowing to peak into its internal structure.
Stereographic views: look at the right picture with your left eye, and vice versa to see the Menger sponge in three-dimensions.
The 2d equivalent of the Menger sponge is the Sierpinski gasket.
It can be constructed by repeatedly carving out squares sitting at the nodes of a square lattice that is resized at each iteration. By changing the scaling factor and the size of the squares, one can get many similar yet different fractal patterns.
