Freesmog
Skeptic1938222569
There shouldn't be any way for Fallout to appear on the map in a normal game. It is used in some scenarios (like the Lord of the Balors), and could be added in worldbuilder, but shouldn't spread.
I think is some old versions of the game I let some buildings like Planar Gates have a small chance of melting down and causing fallout though the vanilla civ mechanism, and of the Nuke mechanism of Auric Ascended or a Warp Bubble causing fallout, but that has not been true for a long time.
I'm doing a Full of Resources mapscript. I found the turn it does the Fallout in the same area. Seems as if a fire elemental might trigger it. Only thing I noticed that might trigger it. The good part is once it spread to a few tiles it never spread beyond that many turns later, as I was worried about. It even disappeared in one tile but no further spread beyond the 8 or so tiles afflicted.