The Chicago Academy of Sciences called this the significant cave of the upper Midwest, and it is usually promoted as the jewel box of American caves, which for once is a fair description rather than marketing. It is not large. It is exceptionally well decorated, and the National Park Service made it a National Natural Landmark in 1987.
The cave sits in the southern slope of the east hill of the two Blue Mounds, near Blue Mounds, Wisconsin. The rock it formed in is galena dolomite, named for the lead ore it carries, laid down roughly 488 million years ago in the Ordovician when warm shallow seas covered most of North America and the shells of marine organisms piled up on the seabed.
The cave itself is far younger, opened out over the last million to million and a half years by a single large crack in the surface rock that the guides call the lifeline. Rainwater picks up carbon dioxide from the air and the soil, turns into a weak carbonic acid, and works its way down that crack. It is not aggressive stuff, but given a million years it dissolved out the chambers, and when the water table dropped they drained and filled with air. That is when the decoration began, because every drop that comes through now leaves a trace of mineral behind.
It was found the same way a lot of caves are found, by people blowing up the hill for something else. Quarry blasting on the Brigham farm broke into the cave in 1939, and the quarry never reopened.
The cave keeps a steady 50 degrees Fahrenheit. Tours are guided and take about an hour on a lit, paved route, and the site is open all year.
Other places on the map within a couple of hours.
