Meramec Caverns

Meramec Caverns is the collective name for a 4.6-mile (7.4 km) cavern system in the Ozarks, near Stanton, Missouri. The caverns were formed from the erosion of large limestone deposits over millions of years. Pre-Columbian Native American artifacts have been found in the caverns. Currently the cavern system is a tourist attraction, with more than fifty billboards along Interstate 44 and is considered one of the primary attractions along former U.S. Highway 66. Meramec Caverns is the most-visited cave in Missouri with some 150,000 visitors annually.

Cave data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).