Mammoth Cave — Gothic Avenue

The world's longest known cave system held a tuberculosis hospital in the 1840s — patients brought underground for the stable temperatures, most of whom died there. Their spirits are the most documented in the cave system, concentrated in Gothic Avenue where the hospital ruins still stand. Floyd Collins, a cave explorer who became trapped and died in a nearby cave in 1925 in the most covered news story of the decade, is said to roam the main cave corridors. Rangers on overnight duty consistently report footsteps behind them in empty passages and voices in sealed rooms.