Sierpinski sponge

Sierpinski sponge (or Sierpinski cube) is the exact three dimensional extension of the Sierpinski carpet, a fractal where one central cube is removed at each recursion. It is slightly different from the Menger sponge, where seven cubes are removed at each iteration (one in the center and one on each face). Because in the Sierpinski sponge only interior volume is removed, there are no exterior holes and the internal structure cannot be visualized unless the cube is partially transparent.