Space-filling Fractal Sponge

This is a "fractal sponge" solid, based on the design on page 61 of the "Alt.Fractals" book. It's closely related to the Menger Sponge, but //this// version "tesselates", it perfectly tiles 3D space with units of a single size: the "proper" Menger sponge doesn't. 80mm.

The shape is discussed at:
 http://alt-fractals.blogspot.com/2011/03/tilable-space-filling-fractal-sponge.html