Crystal Cave is Wisconsin’s longest known cave. Crystal Cave located in Wisconsin's Pierce County, less than a mile west of Spring Valley on State Highway 29. Discovered by accident in 1881 by William Vanasse, Crystal Cave is a multi-level solutional cave formed in dolomite bedrock located in the Prairie du Chien grouping. The Dolomite was formed approximately 485 million years ago in the lower Ordovician period when a warm shallow ocean covered much of Wisconsin and Minnesota.