Crystal Cave near Spring Valley in western Wisconsin is the state's longest known cave, extending about 4,600 feet through dolomite of the Prairie du Chien Group. Discovered in 1881 by William R. Vanasse and first explored with his brother George, it formed as slightly acidic water enlarged joints and fractures in ancient rock laid down about 485 million years ago. The cave is known for a three-level maze structure and a wide variety of formations, including stalactites, stalagmites, flowstone, helictites, and cave pearls. Opened to the public in 1942, it later became an important site for further cave exploration and remains both a tourist attraction and a scientific resource.