What are the Toynbee tiles?

"The Toynbee tiles (also called Toynbee plaques) are messages of unknown origin found embedded in asphalt of streets in about two dozen major cities in the United States and four South American capitals. Since the 1980s, several hundred tiles have been discovered. They are generally about the size of an American license plate (roughly 30 cm by 15 cm), but sometimes considerably larger. They contain some variation on the following inscription:

TOYNBEE IDEA
IN MOViE `2001
RESURRECT DEAD
ON PLANET JUPITER

Some of the more elaborate tiles also feature cryptic political statements or exhort readers to create and install similar tiles of their own. The material used for making the tiles was long a mystery, but evidence has emerged that they may be primarily made of layers of linoleum and asphalt crack-filling compound.
Articles about the tiles began appearing in the mid-1990s, though references may have started to appear in the mid-1980s."

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