Saw a documentary on these last night, quite interesting.
Toynbee Tiles wikipedia (http://en.wikipedia.org/wiki/Toynbee_tiles)
Wired article (http://www.wired.com/magazine/2011/02/toynbee-or-not-toynbee-tile-finders-try-to-answer-the-question/)
Interesting, has the "crop circle" feel to it.
I toyed with the idea of making up some of these and burying them in strategic places. I like the random weirdness of them.
But it looks kind of exhausting so I didn't.