Ever seen one of these?

[miajag]

They're called "Toynbee Tiles." I took that picture yesterday at the intersection of Chase and Cathedral streets in Baltimore, but there are hundreds in cities across the US and even reportedly in South America and Europe. Their origin and meaning is a mystery.

Weird, mysterious cultural phenomena like this (Kilroy was here is another good example) have always deeply fascinated me. What are peoples' motivations for creating them, or if not the original creator, participating in them?

Has anyone here ever been a part of something like this? I'd be interested to hear any stories. Also feel free to post other similar examples of weird things you've seen in different places.