A Toynbee tile is a license plate-sized marker found in asphalt (typically on a busy city street) containing a cryptic message—some believe one man is behind them. The plaques started appearing like crop circles in the 1980s, spurring a slew of conspiracy theories and, eventually, an investigative documentary in 2011:
The people at Tribeca Film Festival just discovered one at Greenwich Street and North Moore, which was still covered up. As noted on Wikipedia, it's believed they are put in place via linoleum, asphalt glue, and tar paper—the latter bakes it in to the street over time. When they peeled back the tar paper these words were uncovered: "Toynbee idea / Resurrect dead / Movie '2001' / Planet Jupiter." Did some intergalactic plan just get set in to motion? Maybe! From TFF:
"Jon Foy's 2011 documentary on the subject, Resurrect Dead: The Mystery of the Toynbee Tiles, posed a potential tiler who, inspired by Stanley Kubrick's 2001: A Space Odyssey and British historian Arnold J. Toynbee, theorized a human colony on Jupiter made up of rehabilitated dead souls. As Foy investigates in his documentary, the man had been rejected by multiple news outlets when he tried to spread his message and, in a more grassroots approach, hit the streets with his tiles."
In 1999, the NY Times reported on the tiles, and talked to one man who was tracking them, as they were popping up in cities all over the world—"He wonders, how does someone 'that far off his rocker get to at least four different major cities and manage to place these plaques securely in the asphalt of the newly paved roads without getting noticed or killed?' He offers a psychological profile of the medallion maker: probably British; possibly lying low in Dover, England, as one plaque suggests; and possessing the financial and intellectual wherewithal to travel and install the signs." Banksy?