A crystal sculpture consists of infinitely many cubes.

Start with a cube with side length of 1.  Then，at the center of each face，attached a cube with side length of 1/3 (so that the center of a face of each attached cube is the center of a face of the original cube).

Continue this procedure indefinitely: at the center of each exposed face of a cube in the structure，attached (in the same fashion) a smaller cube with side length one-third that of the exposed face.

What is the volume of the entire crystal sculpture?