Which UNESCO World Heritage Site in the United States is the world’s longest known cave system?

Mammoth Cave National Park is the UNESCO World Heritage Site containing the world’s longest known cave system.

Mammoth Cave is a network of limestone passages beneath Kentucky’s Green River region. Water dissolved the limestone over immense periods, creating shafts, chambers, narrow canyons, and winding underground routes. More than 685 kilometres of passage had been surveyed by 2022, and exploration continues.

People used parts of the cave thousands of years ago, including for extracting minerals. In the 19th century it became a commercial tourist attraction, and enslaved African Americans played important roles as guides and laborers. The national park was established in 1941.

The UNESCO property includes the cave and a surface area whose forests and river landscapes help protect the underground system. “Mammoth” refers to the cave’s enormous extent, not to woolly mammoths living there. UNESCO inscribed the national park in 1981.