An art student I know did this at my school (U of Calgary) about three years ago. The design is invisible unless it rains or you splash a bucket of water on the

An art student I know did this at my school (U of Calgary) about three years ago. The design is invisible unless it rains or you splash a bucket of water on the wall. It's also in a low-traffic area, so I don't think very many people know about it.