Mammoth Cave is a part of Mammoth Cave National Park in central Kentucky. Mammoth Cave is the world's longest known cave system with more than 400 miles (640 km) of surveyed passageways, which is nearly twice as long as the second-longest cave system, Mexico's Sac Actun underwater cave.
Mammoth Cave developed in thick Mississippian-aged limestone strata capped by a layer of sandstone, which has made the system remarkably stable.
It is known to include more than 400 miles (640 km) of passageway; new discoveries and connections add several miles to this figure each year. Mammoth Cave National Park was established to preserve the cave system.
According to en.wikipedia