Meramec Caverns is an 4.6-mile (7.4 km) cavern system in the Ozarks, near Stanton, Missouri, USA. It was 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 caverns are a tourist attraction, with more than fifty billboards along Interstate 44. It is considered one of the primary attractions along former U.S. Highway 66. Meramec Caverns is the most-visited cave in Missouri the some 150,000 visitors annually. Meramec Caverns is ranked #171 on CaverBob.com's USA Long Cave list.
The Caverns have existed from the past 400 million years, slowly forming through deposits of limestone. Centuries ago, Native Americans used the cavern system for shelter. The first cave west of the Mississippi River to be explored by Europeans, it was "discovered" in 1722 by a French miner. During the 1700s, the cave was used for extracting saltpeter for the manufacture of gunpowder. In the Civil War era, the Union Army used the caves as a saltpeter plant, but the plant was discovered and destroyed by Confederate guerrillas, including future famous outlaw Jesse James. Reportedly, James and his brother and partner in crime Frank used the caves as a hideout in the 1870s. One legend claims that the sheriff sat in front of the cave, waiting for Jesse and his pals to emerge; however, they had found another exit. In 1933, the extended cave system was discovered, revealing the present 4.6 miles (7.4 km), and was introduced to the public as a tourist attraction in 1935 by Lester B. Dill, who invented the bumper sticker as a means of promoting the caverns.
In 1960, Meramec Caverns began to rent billboard space—in the caverns. The owners claimed this was the only underground billboard in the world.