Via Springwise:

GreenGraffiti creates advertising ondirty city streets and walls using the clean, green power of plainwater. Armed with just a template and a high-pressure water sprayer,the company has "cleaned" advertising messages out of the dirt onbehalf of clients including Elle, Telfort and Universal Music. Nopaper, no ink, no printing process—GreenGraffiti's ads are completelycarbon-neutral, it says. They last up to six months, depending on foottraffic, 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!