How does a city of 75,000 inhabitants, spread out over 60,000 acres, make itself invisible? How can familiar place names suddenly disappear from official maps?The mystery began when the famed theoretical physicist Albert Einstein signed a letter, along with other leading scientists, to President Franklin D. Roosevelt in 1939 urging him to launch an all-out effort to build an atomic bomb before the Germans, who marching toward the super weapon, could claim the final strategic victory in World War II. The result, according to historian Ray Smith, was the ultra-secretive creation of Oak Ridge, Tenn., as America’s central facility for beating the Nazis to the atomic bomb. A veteran of 43 years of management service at the Y12 National Security Complex, Smith is recognized as the foremost historian of the genesis and operation of the facilities, nestled in the valleys of Anderson County along the Clinch River, that produced the first atomic bombs, prompting the Japanese surrender and the end of a war that could otherwise have claimed thousands more American lives.