Crystal Cave is a cave located in Wisconsin’s Pierce County, near the Town of Spring Valley on Highway 29. The cave was discovered in 1881 by local brothers George and William Vanasse. Crystal Cave is a multi-level solutional cave formed in dolomite bedrock in the Prairie du Chien Group. The dolomite was formed 485 million years ago during the Lower Ordovician Period when a warm shallow ocean covered much of Minnesota and Wisconsin. It is Wisconsin’s longest known cave.