Longest cave system

What to remember

Beneath south-central Kentucky, near Brownsville, the surveyed passages of Mammoth Cave add up to 685.6 km. No other known cave system is that long.

A key date is 1972, when a connection was found between Mammoth Cave and the even longer system under Flint Ridge to the north. Since then the whole network has formally been called the Mammoth-Flint Ridge Cave System, and it is over 1.5 times longer than the second longest known system, Mexico's Ox Bel Ha underwater cave.

Mammoth Cave National Park takes in part of the system, with the Green River running through it. It is also a World Heritage Site, an International Biosphere Reserve and an International Dark Sky Park. The name refers to the cave's size, not to the prehistoric animal.