Mammoth Cave National Park, Kentucky

Located in Kentucky, Mammoth Cave is the world's longest known cave system.
Mammoth Cave officially became a national park in 1946 — but artifacts found within prove that Native Americans had been exploring this cave system at least 4,000 years before.
Mammoth Cave, located in central Kentucky, is the world's longest known cave system, stretching over 426 miles (686 km) of explored passageways—and more is being discovered regularly. It is part of Mammoth Cave National Park, a UNESCO World Heritage Site and an International Biosphere Reserve.