A few months ago, "reverse graffiti" artist Moose traveled to San Francisco where he created a lovely mural by cleaning grime from the walls of San Francisco's Broadway Tunnel. Moose calls himself a "professor of dirt." Documentary filmmaker Doug Pray (Scratch, Hype!, etc.) made a short film about the artwork. It seems to be a bit of viral propaganda for a "green" cleaning supply company, but it's still a great mini documentary.