The longest known cave system in the world Mammoth Cave National nature Park in Kentucky USA

Mammoth Cave National Park is a national park of the United States in south-central Kentucky. It encompasses portions of Mammoth Cave, the longest known cave system in the world.

The park's 52,007 acres (21,046 ha) are located primarily in Edmonson County, with small areas extending eastward into Hart and Barren counties.

As of 2025, more than 426 miles (686 km) of passageways had been surveyed, over 1.5 times longer than the second-longest cave system, Mexico's Ox bel ha underwater cave.