Ossario: Reverse Graffiti Favorite 

Visual artist Alexandre Orion removes soot in an artful manner to produce skulls in the tunnels of the polluted city of Sao Paulo. Orion does so in an attempt to create intervention, raise awarenes, and decrease pollution. "From the ocular cavities of so many dead, his work looks out on the living and interrogates people passing by; it quietly criticizes our omission, our comfortable acceptance of pollution." ( Jose De Souza Martins)

His works have gotten the attention of the government, although they have destroyed his art they have heard his message of change. So, the government decided to 'clean' the tunnels.

Video:
http://www.alexandreorion.com/ossario/video.html