We will look at four types of urban geosites with a subterranean aspect in the Minneapolis-St. Paul metropolitan area (called the “Twin Cities”) of Minnesota in the American Midwest. Springs, as shown by Coldwater Spring, a unit of the National Park system. Natural caves, as shown by Carver’s Cave, an early landmark on the Upper Mississippi River. Artificial caves, as shown by the abandoned sandstone caves of Mushroom Valley. Sandstone tunnels, as shown by Mill Ruins Park and Pillsbury Park, on opposite banks of the river. However, the monotonous uniformity of the St. Peter Sandstone, in which they are found, raises questions about the meaning of geodiversity. I argue that the subterranean Twin Cities provides a corroboration of the “Ollier argument” (2012) as it applies to geoheritage sites in uniform sandstones. The acute difficulty of balancing science, education, and geoconservation at urban sites like this is also discussed.