The World's Longest Cave System

Mammoth Cave in central Kentucky is the longest known cave system in the world, with over 400 miles of explored passages and estimates suggesting that hundreds of miles more remain undiscovered. The caves were mined for saltpeter, a key ingredient in gunpowder, during both the War of 1812 and the Civil War. Wooden pipelines built by enslaved workers to move water through the cave during the War of 1812 are still visible today, preserved by the cave's constant 54-degree temperature and lack of humidity.

Source: Wikipedia