Visit Moscow's underground

This one might seem a bit weird, because it's not one of the places that we usually like to see when we're visiting a city... but Moscow's underground is different. According to the communist system that there was in the USSR, the beautiful buildings and the luxury had to be where everyone could see it and enjoy it, and what place is there more transited than the undergroud?
So, another of the things I'd like to do in my life is visiting this special and elegant underground.