Though no one really knows who created, or exactly why this strange form of street art exists, many believe the mystery has been solved by Justin Duerr, and his amazing documentary Ressurect Dead.
The story goes that, for over 30 years, one Philadelphia man has used linoleum tiles, and an asphalt compound, to spread his near-schizophrenic message of life-after-death, through a hole in the floor of his car. By placing the tiles onto the street to be constantly run over, the tiles would be pushed into the road to create an almost permanent display of his ideas.
Staring in Philadelphia in the early 1980s, and spreading throughout the Northeast in the 90s (as far south as Washington D.C., and as north as Boston, MA), a few have also been spotted in the Midwest (such as Missouri), and even one in South America - with New York having the largest collection outside of Pennsylvania.
The messages, though brief, are thought to originally stem from the book, Experiences, by British philosopher Arnold J. Toynbee, and mixed with some sci-fi from the 1968 movie 2001: A Space Odyssey, which was written by Stanley Kubrick and Arthur C. Clarke. Nevertheless, the information contained in the tiles has been known to change from time to time, and some even include stories of secret government attacks on the "artist".
It is now accepted that whoever placed the tiles thinks we can colonize the planet Jupiter with the souls of all human life that have died so far.
There is a wonderful interactive map of all the so-far-discovered pieces (link here), which I used to catalog those in New York City.
While there are many spread throughout Manhattan, like the one on the corner of Church and Warren Streets in Downtown...
...most can be found in the Midtown area.
By taking a walk south on Broadway, starting at 42nd Street, then a left on 35th, heading south again on 6th Avenue to 27th Street, and then north on 5th Avenue from 31st to 38th Street, one can find all those pictured below.
Try seeing how many you can spot along your way.
Since the tile work has garnered a bit of infamy, there are quite a few fakes, with varying degrees of competence in the original method.
Happy hunting, and I look forward to someone turning this into some kind of scavenger hunt.