The Toynbee tiles are messages of mysterious origin found embedded in asphalt

Asphalt concrete

Asphalt concrete is a composite material commonly used in construction projects such as road surfaces, airports and parking lots. It consists of asphalt and mineral aggregate mixed together, then laid down in layers and compacted...

 of streets in about two dozen major cities in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 and four South America

South America

South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

n capitals. Since the 1980s, several hundred tiles have been discovered. They are generally about the size of an American license plate, but sometimes considerably larger. They contain some variation on the following inscription:

TOYNBEE IDEA

IN Kubrick's 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.