Planning a Visit? Mammoth Cave in Kentucky is the world's longest known cave system, with more than 400 miles explored, and one of the oldest tour attractions in North America. Mammoth Cave National Park preserves the cave system, a part of the Green River Valley, and the rolling hills of south central Kentucky.

The biggest cave in the United States—and indeed the longest known cave system in the entire world—is Mammoth Cave, located in Mammoth Cave National Park in Kentucky. As of early 2026, explorers have mapped and surveyed over 426 miles (685 km) of subterranean passageways, and new discoveries are made almost every year. The system is a massive limestone labyrinth featuring giant chambers, vertical pits, and underground rivers. While it holds the record for length, if you are looking for the "biggest" in terms of a single open chamber, the "Big Room" in Carlsbad Caverns, New Mexico, takes the prize; it is a single cave chamber that covers 8.2 acres (3.3 hectares) and is nearly 255 feet (78 meters) high. However, for sheer scale and total volume of interconnected tunnels, Mammoth Cave remains the undisputed giant. It is a UNESCO World Heritage site and an International Biosphere Reserve, drawing hundreds of thousands of visitors annually to explore its "grand, gloomy, and peculiar" depths.