The bedrock is assumed to be beneath 6 yards of soil. Human engineers built brick sewers on top of the topsoil--marked in gray and green routes on the map. Manholes are green dots, and streets overhead are beige. I made this map in Inkscape, putting each element on its own layer. So the streets are on its own layer, the subterranean waterways on another, tertiary drainage tunnels (a mere yard across) are on their own layers.
I love using Inkscape for maps because it's so easy to create layers of information. I want to do a better job of making and finding universal design elements like chairs, tables, and so on, in order to make cooler and less industrial maps. In the meantime, I get to work on a fun project that helps us keep track of the campaign and all its complexity.