The exploration of Jewel Cave began in about 1900, when two South Dakota prospectors, Frank and Albert Michaud, and a companion, Charles Bush, happened to hear wind rushing through a hole in the rocks in Hell Canyon. After enlarging the hole, they discovered a cave full of sparkling crystals. The entrepreneurs filed a mining claim on the "Jewel Lode," but they found no valuable minerals, so they attempted to turn the cave into a tourist attraction. The business was never a success, but the cave did attract attention. In 1908, President Theodore Roosevelt established Jewel Cave National Monument to protect this natural wonder.

A half-century later, exploration of the cave intensified. Led by the husband and wife team of Herb and Jan Conn, spelunkers explored and mapped miles of passageways.

When first asked to consider a trek below the surface, the Conns were reluctant. But after their first excursion into the underworld, the couple was hooked. In more than 2 decades of spelunking in Jewel Cave, the Conns logged 708 trips into the cave and 6,000 hours of exploration and mapping. Their efforts proved that Jewel Cave was among the most extensive and complex cave ecosystems in the world, filled with scenic and scientific wonders.

The explorers discovered chambers with exquisite calcite crystals and other rare specimens. One room mapped by the Conns, the Formation Room, is now a highlight of the Park Service tours. They also found rooms as large as 150 feet by 200 feet, passageways as long as 3,200 feet, and a place where the cave wind blows at speeds of over 30 miles per hour. In 1980, after discovering more than 65 miles of passageways, the Conns retired, and a new generation of spelunkers have pushed the known boundaries of the cave to well over 150 miles.

When the Conns said, "We are still just standing on the threshold," they could not have known how accurate they were. Studies by the U.S. Geological Survey have since attempted to determine the number of passageways in the cave by measuring the volume of air leaving or entering the cave, depending on the barometric pressure outside. Results indicate that known passageways at Jewel Cave constitute less than 5% of what actually exists in the quiet darkness below the Black Hills.

Only Mammoth Cave in Kentucky is longer than Jewel Cave. More recent explorations of Jewel Cave have determined it is the second-longest cave system in the world.

Known for its calcite nailhead and dogtooth spar crystal, Jewel Cave is also home to a variety of rare and unusual cave formations. The Cave's hydromagnesite "balloons," fragile silvery bubbles that look as if they might pop any minute, have been found in just a handful of other caves. Scintillites, reddish rocks coated with sparkling clear quartz crystals, were unknown until they were discovered in Jewel Cave. One particularly intriguing mineral, gypsum, combines with time and the ceaseless presence of seeping water to assume the shapes of flowers, needles, spiders, and cottony beards that sway from the heat of an explorer's lamp.