Underneath the rolling sinkhole plains of central Kentucky lies Mammoth Cave, a limestone labyrinth with 412 miles of underground passageways stacked on top of each other in five different levels. It’s the longest cave system in the world, and no one knows exactly how far deep it goes—an estimated 600 miles of passages are still unexplored. A Unesco World Heritage Centre, Mammoth Cave contains every type of cave formation—from icicle-like stalactites to eerie white gypsum flowers—and 130 species of wildlife. Every year, National Park Service guides lead 500,000 visitors through tight passageways, steep shafts and vast chambers that, millions of years ago, were formed by gushing water. Yet without the slave labor of Stephen Bishop, it’s unclear how much of the cave we would know about today.