Mammoth Cave National Park in south-central Kentucky is home to the renowned Mammoth–Flint Ridge Cave System, which is the longest known cave system in the world. The park's area covers 52,830 acres, primarily in Edmonson County, with small parts in Hart and Barren counties. The Green River, together with its tributary, the Nolin River, flows through the park. Since the connection was discovered between Mammoth Cave and Flint Ridge Cave System in 1972, over 420 miles of passages have been surveyed. This is almost double the length of the second-longest cave system - Sac Actun underwater cave in Mexico.