Those cubes inside cubes inside more cubes are amazing. Machining those contraptions from a single piece of stock really is machining skill.
They're also great conversation pieces as people often wonder how they got one inserted into the other, while that was never the case, and it's impossible to remove the inner ones by any non-destructive means.
I'm not sure about practical applications of such things, but they have something about them that makes them feel 'impossible'.