This is Wisconsin's longest known cave, near Spring Valley in Pierce County, and it was found in 1881 by two local brothers, George and William Vanasse.
It is a maze cave on three levels, reaching 69 feet deep and about 4,600 feet long, developed entirely within the dolomite of the Prairie du Chien Group. That rock formed roughly 485 million years ago in the Lower Ordovician, when a warm shallow ocean lay over most of what is now Minnesota and Wisconsin. The third and deepest level is the most extensive, and it grew along a set of joints in the bedrock running northeast to southwest, which is why the passages there follow such deliberate straight lines.
Dating when the cave started forming has proved awkward. Glacial meltwater scoured it and debris was dumped into it during the Wisconsin Glaciation, when the Superior lobe of the Laurentide Ice Sheet ended not far away, and that erased much of the earlier evidence. The accepted explanation is that rainwater and snowmelt picked up biogenic carbon dioxide from the topsoil, became a weak carbonic acid, and worked down existing joints and fractures until they widened into passage. Some researchers argue the origin is hypogenic instead, meaning the water came up from below rather than down from above, which would make it a rather different cave than it looks.
The formations are concentrated in the southeast portion, where the conditions for growth are best.
This is one of two Crystal Caves on the map, and the other one is in Pennsylvania. Tours here are guided, the cave holds around 50 degrees Fahrenheit year round, and the site also runs above ground activities including gem panning.
Other places on the map within a couple of hours.
