What is the world's longest cave system?

The world's longest known cave system is Mammoth Cave in Kentucky, United States.

Its surveyed passages extend for more than 426 miles (about 686 kilometers), although exploration continues and the measured total can grow as new connections are mapped. The system is formally called the Mammoth–Flint Ridge Cave System, reflecting its connection with the nearby Flint Ridge caves.

Mammoth Cave formed in thick layers of limestone shaped by groundwater over millions of years. Its passages include underground rivers, large chambers, narrow crawls, unusual minerals, and evidence of prehistoric human activity. The cave was also mined for saltpeter during the War of 1812.

A common mix-up is confusing the longest cave system with the largest single cave chamber. Vietnam's Sơn Đoòng is famous for enormous chambers, while Mexico's Sistema Ox Bel Ha is the longest known underwater cave system. Mammoth Cave holds the overall length record for surveyed cave passages.