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.

Unlike many of his park-ranger colleagues, DeGraaf is neither a forester nor a geologist but a historian of technology. The passageways of his prized grotto, some 15 feet below the barren, paved courtyard of an aging laboratory complex, are human-made and lined floor to ceiling with shelves of papers. DeGraaf pulls open the vault's thick steel portal to reveal a collection of some of technology's most fertile germinations: the 3,500 handwritten notebooks of Thomas Alva Edison. Now administered by the U.S. Park Service, the vault is the heart of the Edison Archives, a bomb-resistant bunker built below the famous inventor's laboratory in West Orange, N.J.