Mammoth Cave National Park is a U.S National Park in central Kentucky the longest cave system known in the world located primarily in Edmonson County, Kentucky. With 400 miles(640km) of surveyed passageways, Mammoth cave is far by the worlds longest known cave system, being over twice as long as the second-longest cave system, Mexico's Sac Actum underwater cave. Mammoth Cave developed in thick Mississippian-aged limestone strata capped by a layer of sandstone, making the system remarkably stable. New discoveries and connections add several miles to this figure each year.