Mammoth Cave in Kentucky is the longest known cave system on Earth - more than 400 miles of mapped tunnels stretching beneath the bluegrass hills. Today it draws half a million visitors a year. In 1842, it was used as a hospital.

The Doctor Who Bought a Cave

Dr. John Croghan was a Louisville physician who purchased Mammoth Cave in 1839 for $10,000 - roughly $290,000 today. Croghan had a theory: the cave air, which stayed at a constant cool temperature year-round, might have the power to heal lung disease. At the time, doctors were desperately searching for a cure, and spa-like treatments in fresh, unusual air were in fashion across Europe. Croghan believed he had something better than any mountain resort. He had a cave.

A Hospital One Mile Underground

In the autumn of 1842, between 16 and 20 patients entered the cave on foot, carrying their belongings into the dark. Enslaved workers had built eight wooden huts and two stone cabins about a mile inside the entrance - each measuring 12 by 18 feet, with tongue-and-groove floors and canvas roofs. The patients moved in. Lanterns burned around the clock. A guide later recalled the scene: "There were fifteen of them and they looked more like a company of skeletons than anything else."

What Actually Happened

The cave air did not heal anyone. As winter deepened, the enclosed space filled with smoke and ash from lanterns and fires. The damp, still air irritated already damaged lungs. One patient wrote that his lungs were "constantly irritated with smoke." At least five patients died inside the cave, including a New York reverend named Charles Marshall. Nearly all of those who made it out died within days or weeks of returning to the surface.

The Doctor's End

Dr. Croghan ended the experiment in early 1843. The survivors returned to the surface. On January 11, 1849, Croghan died from the same illness he had tried to cure. He was 58 years old.

What You Can See Today

The stone huts Croghan built are still standing inside the cave. Visitors on the Violet City and Star Chamber tours can walk past them today. They are among the strangest relics in the American national park system - the remains of a hospital that sits in total darkness, deep beneath the Kentucky hills, in the longest cave on Earth.

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