I still need to get around to III and IV, but the original was truly a seminal piece of software. Anyway, the reference was to the fact that when you nuked a city in Civilization or Civilization II it would put pollution in a randomly determined number of surrounding squares. This was bad because not only did it lower the production of those squares, but once you got a square with pollution it started a timer, and if you didn't clean up it would ultimately cause your planet to globally warm, turning rivers into plains and plains into deserts. Usually I'd drop a nuke on someones capital, because then it was really cheap to just pay their other cities to switch sides, and because one citys worth of pollution could be cleaned up by a few settlers in a few turns. Do it to 20 or 30 cities though and unless you had a LOT of settlers in the area you were pretty much screwed.