Discovered in 1939, Cave of the Mounds was designated a National Natural Landmark by the United States Department of the Interior and the National Park Service in 1988. Guided tours of the cave are available year-round that take visitors past a varied collection of colorful stalactites, stalagmites, columns and other formations. The main cavern began forming more than a million years ago as acidic water dissolved the limestone bedrock far below the surface. Visitors can avail of various nearby amenities, such as park-like grounds with picnic areas, walking trails and rock gardens. Cave of the Mounds takes its name from the Blue Mounds, two large hills that are landmark features located in Wisconsin. Cave of the Mounds is located in Blue Mounds, Wis.