The Mammoth Cave National Park, located in south-central Kentucky, USA, is the world’s longest known cave system, with more than 400 miles of surveyed passageways. It is home to an incredible variety of geological formations, including stalactites, stalagmites
From Wikipedia
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. The Green River runs through the park, with a tributary called the Nolin River feeding into the Green just inside the park.