Called a "grand, gloomy and peculiar place" by early cave guide Stephen Bishop, a slave who eventually gained his freedom, the world's longest known cave system earns its name honestly.
Located in the hill country of south central Kentucky, Mammoth Cave National Park has more than 400 miles of explored and mapped caves, and more are being documented every year.
Much of the early credit goes to enslaved and eventually free African-Americans, who explored and led tours through the caves throughout the 19th and 20th centuries.
Frozen Niagara is one of a few guided tours to welcome small children, the elderly and people who can only walk short distances, while others are more challenging.
