Cave of the Mounds, near Blue Mounds in southern Wisconsin, is a limestone cave celebrated for the variety and color of its speleothems, including stalactites, stalagmites, flowstone, helictites, and rare cave pearls. Its surrounding rock formed about 488 million years ago in shallow Ordovician seas, while the cave itself began taking shape about 1 to 1.5 million years ago as acidic water dissolved the limestone and later left air-filled chambers where calcite deposits slowly built formations. Discovered accidentally by quarry blasting on August 4, 1939, it was quickly protected, opened for tours in 1940, and in 1987 was recognized as a National Natural Landmark.