The child-engineers of the Undercity agree that the Overcity must be falling apart, what with all the broken pipes, ruptured valves and, worst of all, the coal lines exploding at random. Any Undercity kid can tell you the stories: if you don't clean your face before you sleep, they will take you. If you're lucky, you'll come back in a day or two, seeing the Undercity through coal-blackened eyes. If you're unlucky...