Adds underground versions of surface resources; plays nicely with mods that add resources, as well as RSO. Yes, I know this concept has been done already in different mods, but I like to think of mine as a cleaner and more balanced implementation.
Underground resources are accessible only by the underground mining drill, and work like oil in that they lose richness (chance to produce) but eventually reach a minimum value. Underground mining also has a chance to bring up stone instead of the desired resource.
Adds the tech Combustion drilling (and Combustion drilling 2), which unlock the diesel mining drill and the underground (burner) mining drill, respectively. The diesel drill is simply an improved burner mining drill; the underground mining drill allows access to the new underground resources.