One anonymous artist has been leaving strange messages in cities around the world for decades. These messages are forever encased in concrete by their mysterious maker, but even after all these years, no one has come forward to claim the cryptic creations.

No, they’re not part of some viral marketing campaign. These are the Toynbee tiles — a longstanding cultural riddle that has yet to be solved.

Toynbee tiles were first discovered in the late 1980s, but the media didn’t start paying attention until 1994.

Flickr / Inlaterdays

The tiles can be found on streets, sidewalks, and even highways in major cities in North and South America. They’re embedded into the ground, and there’s really no good way to get them out without destroying them. Most cities treat them like public works of art.

The messages spelled out in tile are all similar in tone. They’re also incredibly confusing.

Flickr / Charles Kremenak

The various tiles make reference to everything from specific locations around the world, to pop culture signifiers, to planetary phenomena. As far as anyone can tell, the topics are always pretty random.

The name Toynbee is featured on many of the works, which is how the tiles got their unusual moniker.

Flickr / Otfrom

People speculate that Toynbee either refers to a Ray Bradbury story, or to historian and philosopher Arnold J. Toynbee.

No one knows who created these tiles, although James Morasco of Philadelphia was the prime suspect for quite some time.

Flickr / Matt

Morasco previously spoke to the press about the colonization of Jupiter after reading a book by Arnold Toynbee. However, Morasco passed away in 2003, and newer tiles have appeared around the world since then.

Some tiles make veiled political statements, and all of them feature the same poor handwriting.

Flickr / William Beutler

Though no one has any idea how the tiles are made, some suspect that they are composed of linoleum and a compound found in asphalt.

Flickr / Cklequ Interior

This tile calls New York City home, and it’s only one block away from the ViralNova office.

The newest tiles were discovered in Delaware County, Pennsylvania, in 2007 and 2008.

Flickr / Dave Riggs

Some tiles showcase slightly different handwriting and colors, which makes some people think that they’ve been made by a copycat. Since no one knows the identity of the original Toynbee creator, however, it’s hard to tell a real one from a forgery.

So far, Toynbee tiles have appeared in over 24 major cities in the United States, and in a handful of cities in South America. Since they’ve been around for over three decades and no one has come forward to claim them, it’s safe to assume that they’ll remain a mystery.