Jewel Cave is now the third largest in the World. It is beautifully decorated and contains calcite crystal spars. Frank and Albert Michaud, two local homesteaders, 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.

As recently as 1959, less than two miles (3 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. 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-foot 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 its the cave.