On August 4, 1939, Cave of the Mounds was discovered accidentally when limestone quarry workers blasted a section of the quarry and exposed an opening to the cave. Work at the quarry then immediately stopped and was never continued. Soon after, the cave was closed to prevent damage from curious souvenir seekers who removed pieces of stalactites and stalagmites to take home. There are still foot-sized holes in the flowstone left by these early explorers. The cave was reopened in May, 1940, after lights and wooden walkways had been installed. Since then, the wooden walkways have been replaced with concrete ones to accommodate the many visitors. Theatrical lighting has also been installed to further highlight the colors and shapes inside the cave. In 1987, the cave was designated a National Natural Landmark by the United States Department of the Interior and the National Park Service. To be considered a NNL, "a site must be one of the best examples of a natural region's characteristic biotic or geologic features."