They're called Toynbee plaques, and they contain the same cryptic messages pressed into intersections in Philadelphia, New York, Washington, Buenos Aires, and Santiago.
They all contain the same message: "Toynbee idea, in Kubrick's 2001, resurrect the dead on planet Jupiter," and the Internet is a hotbed of people with theories about their origin and message, The Chicago Sun-Times reports.
The toynbee.net Web site says they may refer to Ray Bradbury's short science fiction story "The Toynbee Convector."
The site lists eight Chicago intersections to spot the plaques but the newspaper found only two, and then found out why.
"It is considered vandalism," said Brian Steele, spokesman for the Chicago Department of Transportation. "If we see them, we'll look for a way to remove them. We treat it no different than graffiti."
Teacher apologizes for showing sexual image of herself in class
Scarlett Johansson steps out with fiance after pregnancy reveal