Also called "cave pearls", they are a beautiful but rare speleothem. These are spherical formations made when a droplet falls onto some sand and calcite forms around the sand, much like the way a pearl is formed. These formations come in many different brilliant colors such as reds, browns, blues, and grays. The reds and browns are caused by the presence of iron oxide in the formation. Similarly, blues and grays are caused by manganese oxide. Some speleothems are even partially luminescent and give off light for a brief period after exposure to another light. Tourism The cave is located 20 miles west of Madison, off U.S. Highways 18/151 in Blue Mounds, Wisconsin. Since its opening in 1940, the cave has hosted millions of tourists. Footnotes References Ahlers, Joe. "Spelunking for amateurs: Cave of the Mounds; Geological wonder lets visitors get up close and personal with science." Milwaukee Journal Sentinel, 27 July 2006, MKE Edition, Sec. Z, p. 21. External links Cave of the Mounds website Caves of Wisconsin Show caves in the United States Limestone caves National Natural Landmarks in Wisconsin Landforms of Dane County, Wisconsin Tourist attractions in Dane County, Wisconsin