Occasionally, adventurers would come to test their limits in the cave. In 1863 the stoic, risk-averse Nick Bransford was somewhat comically paired with a brassy Englishman named F. J. Stevenson, and the two went on a series of misadventures. For the pleasure of his client, Nick scrambled across precarious ledges, illuminating the cave’s remote crags, while Mr. Stevenson watched in gleeful horror, until he became so giddy that he beckoned his weary guide down to help him reach the very same spot. The relationship was finally terminated when Mr. Stevenson left Nick to wait on a mud bank for several hours, when they were exploring the cave’s underground rivers. Upon his return, Stevenson noted in a journal that Nick “had made up his mind that I had met with an accident and that he would be left alone to die in the darkness... He swore he would go no more with me on voyages of discovery for love or money.”

Even though the guides enjoyed a certain celebrity underground, it didn’t shield them from the hardships of slavery or life in the years that followed. Nick bought his freedom — partly through money he earned selling the cave’s eyeless fish to tourists. He was thought to have gone to Nashville, but soon he moved back.

As Mr. Bransford said, “He found out that just because you have some document that says you’re free doesn’t mean people will treat you any better.”

The hardest story for him to tell is the one of Mat Bransford, his great-great-grandfather. Before the Civil War was over, three of Mat’s children were sold to slave buyers.

Sometimes, the guides would scrape their names into a wall to show where they had been. Today, when Mr. Bransford gives tours, he always notices the ones that were made by his family. So far, he has counted 14, but the one that gives him pause is in an area known as Gothic Avenue. It says simply, “Mat 1850.”

“Whenever I see a signature from my kin, I feel awed by what they did,” he said. “But when I see Mat’s, it just knocks me down. I don’t know how anyone can have their kids taken away and never get them back.”

Mr. Bransford spent a long time trying to locate firsthand accounts that mentioned his great-great-grandfather. One day he found a rare interview, taken from a journal of a Union soldier and abolitionist, who had taken a liking to Mat and tried to persuade him to go to Nashville, where he could live out his life as a free man. But as the interview says, “the old man said he couldn’t part with the cave.” And even when freedom finally came, the Bransfords never did.