Pittsburgh's Toynbee Tiles

Swoon's work reminds me of another mysterious entity who's street interventions appear in Pittsburgh. You may have walked accross one downtown without noticing it. According to the Wikipedia, people first started noticing strange plaques ( some very disturbing ) embedded in the Philly crosswalks in the early 1980's and now they have turned up in an amazing number of cities in Both North and South America.

Here's a story in Chicago about them and one in Philly and one in Saint Louis.