Toynbee Tiles is the common term for a collection of messages found pressed into the asphalt of several cities in the eastern United States (plus a few outlier Tiles in South America). They first appeared in Philadelphia in the 1980s, then spread to other towns, all featuring the same central message: "Toynbee Idea/in Kubrick's 2001/Resurrect Dead/on Planet Jupiter." Some Tiles feature smaller messages in panels along the side. The letters are carved from linoleum, using asphalt crack filler in the empty spaces, then wrapped in tar paper and pressed into the pavement by passing cars.

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No one knows for certain who put them there or what they mean. The excellent 2011 documentary "Resurrect Dead" dissects the possible meaning of the Tiler's messages, and makes an educated guess at his identity - but he has not confirmed this, nor responded to any communication. A number of copycat artists have followed his technique and continued placing tiles around the country, including at least 3 here in Portland.

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Source: Wikipedia