Subterranean Twin Cities is the very first comprehensive published account of the wide variety of subterranean spaces, such as caves and tunnels, under the Twin Cities of Minneapolis and St. Paul, Minnesota, set in a geological and historical context, based on more than two decades of research by Dr. Greg Brick. http://www.amazon.com/Subterranean-Twin-Cities-Greg-Brick/dp/0816645973 http://www.gregbrick.org/