The walls of our cities are becoming canvases for creative expression in the hands of a new generation of artists. These kids are street-smart and engaged, (and, OK, they’re not all kids), and many of them infuse their work with social and environmental consciousness. In the Writing on the Wall series, Grist’s special projects editor Greg Hanscom looks into this colorful urban underworld, which one observer calls “the ghost in the urban machine becoming self-aware.”