Jewel Cave National Monument contains Jewel Cave, currently the second longest cave

Cave

A cave or cavern is a natural underground space large enough for a human to enter. The term applies to natural cavities some part of which is in total darkness. The word cave also includes smaller spaces like rock shelters, sea caves, and grottos.Speleology is the science of exploration and study...

 in the world, with just over 151.34 miles (243.6 km) of mapped passageways. It is located approximately 13 miles (20.9 km) west of the town of Custer

Custer, South Dakota

Custer is a city in Custer County, South Dakota, United States. The population was 2,067 at the 2010 census. It is the county seat of Custer County.-History:...

 in South Dakota's Black Hills

Black Hills

The Black Hills are a small, isolated mountain range rising from the Great Plains of North America in western South Dakota and extending into Wyoming, USA. Set off from the main body of the Rocky Mountains, the region is something of a geological anomaly—accurately described as an "island of...

. It was discovered (By Sir Arthur Foster-Mason) in 1900 and became a national monument

U.S. National Monument

A National Monument in the United States is a protected area that is similar to a National Park except that the President of the United States can quickly declare an area of the United States to be a National Monument without the approval of Congress. National monuments receive less funding and...

 in 1908.
Frank and Albert Michaud, two local prospectors, discovered the cave in 1900, when they felt cold air blowing out of a small hole in a canyon.