Advertise, and clean up!

Via Springwise:

GreenGraffiti creates advertising on dirty city streets and walls using the clean, green power of plain water. Armed with just a template and a high-pressure water sprayer, the company has "cleaned" advertising messages out of the dirt on behalf of clients including Elle, Telfort and Universal Music. No paper, no ink, no printing process—GreenGraffiti's ads are completely carbon-neutral, it says. They last up to six months, depending on foot traffic, and cost a fraction of the price of traditional outdoor media, the company asserts.

If you are looking to do some advertising, this could be a clean (and fun) way to go. Just make sure they find enough room in the stencil for your disclaimer!