Which National Park Service park contains the world’s longest known cave system?

Mammoth Cave National Park contains the world’s longest known cave system.

The park in south-central Kentucky protects the Mammoth–Flint Ridge Cave System, whose surveyed passages extend for more than 400 miles. Exploration continues, so the recorded length can change as researchers map additional passages.

Mammoth Cave became a national park on July 1, 1941, after lengthy efforts to acquire land and resolve private ownership. Its underground passages formed in thick limestone, where groundwater gradually enlarged cracks and created chambers, tunnels, and underground waterways.

The cave’s name does not mean that woolly mammoth fossils were found there. “Mammoth” refers to the cave’s impressive size. The park was designated a UNESCO World Heritage Site in 1981 and is also recognized as an international biosphere reserve.