They’ve been spotted around the world, and right here in St. Louis. They are mysterious markers, embedded in city streets. What do they mean and how did they get there?

Like street signs and stoplights, you’ve probably passed them dozens of times downtown. Still their message remains a mystery. Passerby Brian Yount jokes, “Maybe it’s a message from space. The aliens are trying to contact us.”

Slightly bigger than a license plate, they are plaques embedded in the crosswalks. This one reads: “Toynbee idea in Kubrick’s 2001. Resurrect dead on planet Jupiter.”

In St. Louis, the mysterious markers sit in several intersections: Market and 8th, Market and 7th, and Olive and 6th. Graphic artist Mark Plattner discovered one of them. He says, “I didn’t realize until the other day that there were more in St. Louis. I thought mine was the only sighting.”

There are numerous websites devoted to deciphering the plaques. They’re referred to as “Toynbee tiles” because of their text.

One internet theory is that they were created by a Philadelphia social worker named James Morasco. He believed historian Arnold Toynbee and filmmaker Stanley Kubrick had figured out how to colonize Jupiter with dead people from earth.

The problem with this theory is that Morasco died in 2003 and the plaques keep popping up.

Plattner has his own theory. He thinks someone is perpetuating the project. He says, “I think it’s a guerilla art project. I think somebody’s just going out, being weird, and enjoying being the mystery artist and the subject of hot debate and conspiracy theories.”

We asked art historian Jeffrey Hughes of Webster University about guerilla art, which he says would be akin to grafitti or street performances. Hughes says, “It carries its message out. It confronts the public, but it’s still somewhat cryptic. The artist knows what they’re doing, but we don’t.”

There are 130 known plaques, most in the U.S., some in South America. The prevailing theory is that they’re made of linoleum, asphalt crack filling compound and tar paper, embedded in the pavement over time.

Mark Plattner finds it fascinating. He says, “Somebody put a piece of art that’s going to last many years in public without getting permits, without any kind of permission. They just came in at night, stuck it down and disappeared.”

The plaques were first sighted in the early 1980s. And most are in New York and Philadelphia. The ones in St. Louis seem to have been around for at least five years. City officials were not aware of their exisitence.