Leonard DeGraaf, sporting the familiar beige and green uniform of the U.S. National Park Service, leads the way through a narrow subterranean passageway to one of the country's invaluable and rarely viewed wonders. Rounding a final turn, DeGraaf points to the chamber before him. "This is always a thrill for me, no matter how many times I come here," he says in the kind of hushed, reverent tone you might expect from a park ranger approaching the rim of the Grand Canyon or spotting a bald eagle. DeGraaf's enthusiasm, however, is directed toward the massively thick steel door of an underground bank vault.

Select from the choices above to read the entire article.