Buried under layers of limestone and sandstone hundreds of feet thick, Jewel Cave is more ancient than South Dakota's Black Hills, which adorn the landscape above. The cave lay undiscovered until 1900, when two brothers, miners and part-time cowboys, felt a strong wind coming from a small hole in the ground at the base of a cliff. When they enlarged the opening, they found passages filled with the glittering calcite crystals that give the cave its name. Although its discoverers marveled at the cave's natural beauty, few believed the find to be significant. Even after Pres. Theodore Roosevelt...