Argentinian artist Leandro Erlich created this illusion of people walking underwater called The Swimming Pool, for The Century Museum of Contemporary Art in Kanazawa, Japan. It's a piece of glass with water on top of it, and empty space on the bottom.
Nagai Hideyuki uses two sketchbooks to create illusions. I was thinking this may be possible to do on the street/buildings with chalk, but in order to view it correctly everyone would have to be the same height, so that probably wouldn't work.
(link) STREET ART: Chalk Art by David Zinn 2 ~ Alien Monster Rake / Raking the Autumn Leaves ~ Fall Harvest / yard garden / work and play / kids child children / concrete sidewalk / path ~ for more great PINs w/good links visit ~ have fun!