Which US national park features the largest cave system in the world?

Mammoth Cave National Park features the largest known cave system in the world: Mammoth Cave in Kentucky. The mapped passages extend for more than 426 miles, although exploration and surveying continue, so the measured total can change over time.

The system became dramatically larger after explorers connected Mammoth Cave with the nearby Flint Ridge Cave System in 1972. It is now formally known as the Mammoth–Flint Ridge Cave System. Its passages formed primarily as groundwater dissolved limestone over immense spans of geological time, creating chambers, tunnels, underground rivers, and distinctive formations.

The park was established on July 1, 1941, and protects about 52,000 acres in south-central Kentucky. A common mix-up is Carlsbad Caverns National Park in New Mexico, which is famous for enormous chambers and spectacular cave formations but does not contain the world’s longest known cave system. Mammoth Cave is also recognized as a UNESCO World Heritage Site and an International Biosphere Reserve.