Which US cave system is the world's longest known cave system?

The Mammoth Cave system is the world's longest known cave system.

It lies beneath south-central Kentucky and is protected within Mammoth Cave National Park. The name can be misleading: “Mammoth” refers to the cave's immense size, not to woolly mammoth fossils. The system is famous for its vast maze of limestone passages, underground rivers and chambers formed by groundwater dissolving soluble rock.

The modern system includes the Mammoth Cave and Flint Ridge networks, which were formally connected in 1972. Exploration and surveying continue, so its measured length changes as new passages are mapped; by 2025, more than 426 miles (about 686 kilometers) had been surveyed. That is why the careful wording is “longest known,” rather than simply “longest cave.”

A common mix-up is choosing Jewel Cave, also in the United States. Jewel Cave is among the world's longest known cave systems, but Mammoth Cave remains substantially longer. Carlsbad Caverns is more famous for spectacular chambers than for holding the length record.