Mammoth Cave National Park

The world's longest known cave system, with over 400 miles of explored passages. It's been a tourist attraction since 1816, and the constant 54-degree temperature and underground atmosphere have made it the setting for both wonder and some genuinely dark history.

In the 1840s, Dr. John Croghan built huts inside the cave to treat tuberculosis patients - all 15 got worse and two died within the first year. Their phantom coughs still echo through the passages. Stephen Bishop, an enslaved guide who mapped much of the cave, is seen leading phantom tours. Floyd Collins, who died trapped in nearby Crystal Cave in 1925, calls for help from beyond the grave. Melissa, a woman who allegedly abandoned a suitor in the cave without a lamp, searches for his spirit. Park rangers have documented over 150 paranormal events.