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Its a mathematical theory, I think.
If you split a cube up like a rubiks cube, you can take out the middle cube of one 3x3 side. This leaves you with a bunch of cubes, wash rinse repeat. Its like Sierpinskis (spelling?) triangle, just in 3D.
Gamewise, theres a HL2 mod called Dystopia that has parts of a level that looks like that.