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.
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 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 2 miles (3.2 km) of passageway had been discovered. That year, however, Herb and Jan Conn, local rock climbers, began exploring, and within two years had mapped 15 miles (24 km). Much of the new discoveries lay outside the boundaries of the monument, under land managed by the United States Forest Service. The two agencies performed a land swap in 1965, establishing the present boundaries of the park, and enabling the development of a new part of the cave. The Park Service sunk a 300 feet (91 m) elevator shaft to a previously remote cave area, and built concrete walks and metal stairs and platforms along a one-half-mile loop. The "Scenic Tour" was opened in 1972. Most modern-day visitors tour that part of the cave. In August 2000, an 83,000 acres (340 km2) forest fire burned 90% of the monument and the surrounding area. The visitor center and historic buildings were spared.
By 1979, Herb and Jan Conn had discovered, named, and mapped more than 64 miles (103 km) of passages. Although they largely retired from caving by the early 1980s, exploration has continued unabated. Because the areas being explored take many hours to reach, explorers now sometimes camp in the cave during expeditions of as long as four days. The cave is mapped by traditional survey techniques, using compass, clinometer and today with lasers instead of tape measures.
Its 181.89 mi (292.72 km) of mapped passageway make Jewel Cave the third longest cave in the world, after Mammoth Cave System in Kentucky and Sistema Sac Actun at the Yucatán Peninsula (Mexico), at 198 mi (319 km).