Menger's Sponge

Menger's sponge (sometimes wrongly called Sierpinski's Sponge) is a fractal solid that can be described as follows. Take a cube, divide it into 27 = 3 x 3 x 3 smaller cubes of the same size and remove the cube in the center and the six cubes that share faces with it. You are left with the eight small corner cubes and twelve small edge cubes holding them together. Now, imagine repeating this process on each of the remaining 20 cubes. Repeat again. And again ...

http://world.std.com/~j9/sponge/menger-sponge.html