ST. LOUIS (AP) - Unless you're hunting for them, the weird markers embedded in downtown streets in St. Louis don't draw much attention.

For those who do notice, the words make little sense.

The shoe box-sized marker read: "TOyNBEE IDEA IN KUbricK's 2001 RESURRECT DEAD ON PLANET JUPiTER.''

The plaques, numbering more than 100 and found on dozens of city streets across the United States and in three South American countries, present a riddle that may never be solved: What in the world - or on Jupiter, for that matter - does English historian Arnold J. Toynbee have to do with filmmaker Stanley Kubrick and with raising the dead?

St. Louis has three of the "Toynbee plaques,'' or "tiles,'' as they are often called. Kansas City has one.

Some have called the markers the urban equivalent of crop circles. Others say they're just quirky underground graffiti, some done by copycats.

No one has ever been caught or taken credit for this caper that dates back to at least the 1980s.

A Web site, www.toynbee.net, one of the best sources of information on the tiles, is dedicated to mapping and discussing the phenomenon.

Chris Clark went to the site after she stumbled across the plaques in the 1990s in St. Louis.

"The pleasure of these plaques is not so much in the solution and 'whodunit,''' Clark said. "It's hearing the wild theories and stories that surround them.''

Some have tried to make the connection to Kubrick and his sci-fi classic "2001: A Space Odyssey.'' Internet searches return a load of theories, but the connection with the late filmmaker is vague, at best.

"The meaning of the message on the tiles is pretty open-ended,'' said Justin Duerr, a Toynbee tile fanatic. "You can draw a lot of connections between the two, depending on how far out you want to stretch it.''

Another variation reads: "TOyNBEE IDEA IN MOViE 2001 RESURRECT DEAD ON PLANET JUPiTER.'' Sometimes, there are other cryptic messages as well.

Duerr lives in Philadelphia, the place where many people believe the strange practice of stamping the message on city streets began. A plaque in Santiago, Chile, lists an "A. Toynbee'' and a Philadelphia address, but the address - while real - provided no solution.

In 1983, an article in The Philadelphia Inquirer told of a man named James Morasco who said Toynbee's theory of bringing dead molecules back to life on Jupiter appeared in Kubrick's 1968 film. Many have argued that no such reference exists, but Jupiter is part of the movie.

Toynbee was best known for his writings on the rise and fall of civilizations.

In the late 1990s, Duerr became obsessed with the idea and the plaques that appear to be melted into the streets like crayons on a hot day.

Last year he developed the Web site www.resurrectdead.com to document the tiles, and he says he plans to make a movie on the subject.

Duerr said even after Morasco's death in 2003, new plaques appeared in Philadelphia and elsewhere.

At one time, Duerr said, the tiles could be found every other block. Many have since been paved over or worn down to nothing by traffic.

Many of the plaques look identical, as if made with a cookie cutter. Others change the wording slightly and take on a colorful style of their own. Some include political messages.

One Web site reported a Toynbee tile in Pittsburgh that offered instructions on how to make the plaques using several layers of linoleum and glue. Another tile reads, "You must make and glue tiles!''

That was enough to persuade Duerr to make a tile of his own and slap it to a street in Philadelphia. He said his version looks almost like the others.

His actions present the most obvious explanation: Other people were intrigued enough by their encounter with the tiles to make their own in various cities.

Duerr said there's even a name for such behavior: A meme, pronounced "meem,'' meaning a cultural action that is transmitted and repeated over and over.

"I don't believe there's a lone gunman,'' Duerr said. "I like to look at it as art that exists for a reason other than being in an art gallery.''