  jewel cave national monument contains jewel cave, currently the third longest cave in the world, with just over  of mapped passageways. it is located approximately  west of the town of custer in south dakota's black hills. it became a national monument in 1908. ==history== 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. it is unknown whether any previous inhabitants of the area were aware of the natural cave opening, which was not large enough for a person to enter.jewel cave brochure; national park service; gpo, wdc frame|left|calcite crystals in jewel cave after enlarging the cave entrance with dynamite, the michauds found a cavern lined with calcite crystals, which led them to name it "jewel cave." the brothers tried to capitalize on the discovery, widening the opening, building walkways inside, and opening it to tourists. although their venture was unsuccessful, news of the discovery eventually reached washington. president theodore roosevelt proclaimed jewel cave a national monument on february&nbsp;7, 1908. the area around the natural entrance to the cave was further developed by the civilian conservation corps in the 1930s. the national park service assumed management of the monument in 1933 and began offering tours in 1939. as recently as 1959, less than  of passageway had been discovered. that year, however, herb and jan conn, local rock climbers, began exploring, and within two years had mapped . much of the new discover
1908_establishments_in_South_Dakota Black_Hills Caves_of_South_Dakota Civilian_Conservation_Corps_in_South_Dakota IUCN_Category_V Landforms_of_Custer_County,_South_Dakota Protected_areas_established_in_1908 Protected_areas_of_Custer_County,_South_Dakota Show_caves_in_the_United_States