It starts the way these things always start, with somebody looking down. A pedestrian in Philadelphia, or New York, or Chicago, notices a patch of asphalt that doesn't look right. About the size of a license plate, linoleum-framed, glued into the middle of a crosswalk or the middle lane of traffic where no sane person would ever stand to place it. And the message, pressed into the tar by years of cars, reads something like: TOYNBEE IDEA. IN MOVIE 2001. RESURRECT DEAD. ON PLANET JUPITER.

That's not a typo. That's the tile.

Atlas Obscura has a good rundown of the whole saga, and I've been sitting with it for most of a night now, the way you do. The tiles started showing up in the mid-1980s, eventually numbering in the hundreds, spread across at least two dozen US cities, with at least one reported as far away as Santiago, Chile. Linoleum and asphalt crack filler, laid at intersections where a car can pause just long enough for someone to drop one out the door and let traffic do the rest. The messages mash together three things that have no business sharing a sentence: Arnold Toynbee, a British historian; Stanley Kubrick's 2001: A Space Odyssey; and the idea that dead human molecules could be reconstituted on Jupiter.

The connective tissue, as far as anyone can tell, is a 1973 one-act play by a Philadelphia social worker named James Morasco, who believed Jupiter's atmosphere could harbor resurrected human life. Whether Morasco ever laid a single tile has never been proven. What the people who dug into this case will tell you is that when they found the person most likely responsible, he shut the door. That's it. No confession, no denial, just a door closing on decades of anonymous pavement sermons.

And that's the part I can't put down. The tile-maker had a theory of the universe. A strange one, stitched together from a historian's philosophy of history and a science fiction film, but a theory, coherent to its owner, labored over somewhere private and laid into highways one anonymous night at a time. I keep circling back to the medium itself, too, because it's perfect in a way no pamphlet or billboard could be. You can't tear a tile out of asphalt. Traffic just grinds it deeper. The city becomes the printing press, every passing car another strike of the plate, until the message is literally fused with the infrastructure...

Was it one person? A small circle of believers? Nobody knows, and I prefer it that way, honestly, because the Toynbee tiles were never a mystery to be solved so much as a message to be received. Someone shouting at the universe through the pavement, convinced that death was a solvable problem and Jupiter was the answer.

Somewhere tonight there's a tile in a crosswalk in Philadelphia that thousands of tires will press a little deeper into the road. Whoever laid it would probably be fine with that. That was the whole point.