Mammoth Cave National Park, National park in Kentucky, United States
Mammoth Cave National Park protects the longest known cave system on Earth, with more than 426 miles (686 km) of surveyed passages as of 2025. Its large horizontal passages, such as the Main Cave seen on the Historic Tour, lie in Mississippian limestone beneath a cap of Big Clifty Sandstone. The sandstone cap keeps many upper passages dry, while rooms like Frozen Niagara hold stalactites and stalagmites where the cap has eroded.
Indigenous peoples used the cave for mineral extraction around 4000 years ago, leaving behind torches and tools in the front passages. European settlers rediscovered the system in 1797 and mined it for saltpeter during the War of 1812 to produce gunpowder.
The name Mammoth refers to the enormous size of the cave system and not to prehistoric animals, which never lived here. Visitors can see names and inscriptions left by early explorers on the walls during guided tours, some dating back to the 19th century.
Most tours last between one and three hours and require different levels of physical effort, from flat paths to steep stairs. Warm clothing is advisable year-round, as the temperature underground stays constant at around 54 degrees Fahrenheit.
Blind cave fish and colorless crayfish live in the underground rivers, having adapted to complete darkness over time. The stable temperature allows rare bat species to hibernate here, with some colonies numbering in the thousands.
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