Near Hannibal, Missouri, Mark Twain Cave is closely linked to Samuel Langhorne Clemens, later known as Mark Twain, whose boyhood explorations there influenced several of his books and inspired McDougal's Cave in The Adventures of Tom Sawyer. Renamed in his honor in 1880, it has operated continuously as Missouri's oldest show cave since 1886. First documented in 1819–1820, the cave is a maze of narrow passages in rare Louisiana Lithographic Limestone, stretching about 3 miles with four entrances. Its long history includes Joseph Nash McDowell's medical experiments, legends of Civil War use, growing tourism, and the 2019 authentication of Clemens's signature on its wall.