Toynbee Tiles are cryptic messages cut from colorful linoleum tiles and melted into roadways. They've appeared in many major U.S. cities and several South American locations, however it appears the artist originated near Philadelphia. Most inscriptions follow a similar pattern:
IN KUBRICK'S 2001
ON PLANET JUPITER.
Paranoia + novel method of expression + bizarre message = art and intrigue. (I listed this post in the self-aware robots must be stopped category because the tiles make direct referrence to 2001 A Space Odyssey, perhaps warning us of an impending robotic doomsday)