The Cave of the Mounds is a cave system located on Brigham Farm, in Blue Mounds, Wisconsin. It is made from 400 million-year-old limestone from compacted seashells from when modern-day Wisconsin was covered in a shallow ocean. After the water receded, rainwater that combined with carbon dioxide worked its way down to the limestone. This mixture created a weak carbonic acid that slowly eroded the cave. Surface water seeping into the cave as the water table of streams lowered also helped form the Cave of the Mounds. Over time, these erosional processes made different rock formations like stalactites and calcite crystals. With no natural entrances, the cave was discovered in 1939 during a routine blast in the limestone quarry located on the property. After its discovery, the Brigham leased the Cave of Mounds to two businessmen who made the cave accessible to become a tourist attraction. In 1988, the Department of the Interior and the National Park Service designated the Cave of the Mounds a National Natural Landmark.