In my current game I have no coal on my own territory. I already went to war with my neighbouring civ over their two squares of saltpeter, when I had none; so I decided to go to war with them again over their coal. (no one would trade coal with me.) So I disrupt my perfect civ to go to war, and spend many turns working my way across the other civ's territory, patiently conquering cities and quellig rebellions, and fending off counterattacks. Many turns later I get to the enemy city where earlier I saw their coal (after getting their worldmap and using CTRL-SHIFT-M to make doubly sure) - and their bloody coal was gone. Gone! I used CTRL-SHIFT-M again and had a quick look around. The coal had vanished from that square, and another seam had appeared.... three squares into the territory of my neighbouring civ on the OTHER side. Bloody hell, *&^%$^*^, I thought calmly.