I don't know what you think of them. Are they really encoded messages from super intelligent alien races? I certainly hope so but...why didn't they just post on the internet? Or send a party invite.
No I'm afraid it is far more likely that these wonderful examples of graffitti are part of a guerilla art movement probably arising from all those children in the 60's who were bought spirographs for Christmas. These lovely images are from the spirograph website.
You can see the fairly obvious connection? Personally I think they're beautiful examples of art. Keep em' coming! This uncredited picture of a crop circle on Silbury Hill is from the website you can find at the link below. This circle has a very Aztec feel to it.