Mark Twain Cave — originally McDowell's Cave — is a show cave located near Hannibal, Missouri, USA. It is the oldest operating show cave in the state, giving tours continuously since 1886. Along with nearby Cameron Cave, it became a registered National Natural Landmark in 1972, with a citation reading "Exceptionally good examples of the maze type of cavern development". Mark Twain Cave — as "McDougal's Cave" — plays an important role in the novel The Adventures of Tom Sawyer (1876) by Mark Twain and was later renamed in honor of the author, a Hannibal native.Early historyGeologyGeologically Mark Twain Cave and its nearby neighbor Cameron Cave differ from most of the 6,500+ caves found in Missouri. Both are believed to be remnants of a much larger cave system cut apart by a glacier and millions of years of erosion, leading to speculation by geologists and common citizens alike that there may be as yet further undiscovered caves in the Hannibal region. This speculation was heightened in 2006 when the entrance to a previously unknown cave was found during construction of a new elementary school. Other differences in Mark Twain Cave are the near total lack of speleotherms, mineral deposits like stalagmites and stalactites in large open areas. Mark Twain Cave and Cameron Cave instead have a multitude of narrow, winding passages. The caves are made mostly of a soft limestone called Louisiana Lithographic Limestone and found only in a 35-mile area around Hannibal and Louisiana, Missouri. The limestone has been estimated by geologists as around 350 million years old, while the cave passages were formed some 100 million years ago. Mark Twain Cave covers some six and a half miles consisting of four entrances and 260 passages, maintaining a year around temperature of 52 degrees Fahrenheit.
