OAK RIDGE, Tenn. - The poster's hush-hush tone said it all: "What you see here, what you do here, what you hear here, when you leave here, let it stay here."
Here in the rolling hills of eastern Tennessee, America built a secret city. It was one of three such places that developed the atomic bombs dropped on the Japanese cities of Hiroshima and Nagasaki, bringing an end to World War II.
The Tennessee city, now called Oak Ridge, once was home to 75,000 people, yet it did not appear on any map. Visitors could get into the town only through gated entrances.
The vast majority of residents were unwitting participants in the drive to harvest enriched uranium for the "Little Boy" bomb that devastated Hiroshima on Aug. 6, 1945. They learned exactly what they were doing only when they read the big, black headlines that proclaimed the war's end.
Now, 66 years later, Oak Ridge marks its heritage with an annual Secret City Festival, two days of family entertainment each June. Year-round, the town presents an array of museums, historic sites and buildings that convey what happened behind a curtain of secrecy hard to fathom in the age of Google Earth, Facebook and Twitter.
"These days you could never do that," said Bill Wilcox, 88, the city historian, who experienced Oak Ridge's deception from the moment he arrived here as a chemist fresh out of college.
After telling Wilcox he would be working with uranium, his superiors ordered that he never utter the word — at least until the war was over.
As visitors learn from the signature exhibit at the American Museum of Science & Energy here, Oak Ridge was born in 1942 as America raced to build an atomic bomb before Hitler's Germany. The effort, code-named the Manhattan Project, built two other secret cities: one in Hanford, Wash., where plutonium was made for the bomb dropped on Nagasaki; the other, in Los Alamos, N.M., where the bombs were assembled.
The U.S. Army Corps of Engineers snapped up 59,000 acres of eastern Tennessee farmland. A thousand rural families were given as little as two weeks to leave. They would make way for an instant city and four atomic plants, including a U-shaped, 44-acre behemoth called K-25 that measured half a mile by 1,000 feet.
The army picked Oak Ridge for several reasons: Its temperate climate would allow for year-round construction. Nearby dams would provide the needed electricity. The area's landscape — long valleys framed by parallel ridges — would shelter the plants from spies. And if one of the plants blew up, the ridges would prevent an explosion from spreading like a string of firecrackers.
The town grew like kudzu, zooming past the initial projected population of 13,000 because so many workers were needed. In less than four years, by Wilcox's count, the Army engineers had built nearly 10,000 homes for families, 90 two-story dormitories, 5,000 trailers, barracks and huts for 16,000 people, a dozen shopping centers, nine neighborhood schools, two chapels and the nation's ninth-largest bus system.
It was called the "Clinton EngineerWorks," a bland name that would not attract attention. When nosy people from nearby Knoxville asked what was going on inside the gates, they were told, "Oh, we're just building a bunch of homes for all the officers to come retire in after the war," Wilcox said.
Today, especially along its franchise-lined commercial strip, Oak Ridge looks more like any city than Atom City. But visitors can still glimpse vestiges of the town's storied past.
An outdoor exhibit at the American Museum of Science & Energy features one of Oak Ridge's prefab "flat-top" houses: a 567-square-foot two-bedroom, complete with two sinks (one for washing clothes, the other for dishes). The homes were trucked into town in two halves, completely furnished.
Nearby is Jackson Square, the historic arcaded shopping area that once hummed 24/7 as townspeople frequented its movie theater, bowling alley, dance hall and stores. You also can see the shuttered, classically dressed Guest House, which once hosted renowned nuclear physicists who signed in under assumed names, and the white, spire-topped Chapel on the Hill, which hosted multiple religious denominations during the war.
Elsewhere, you will encounter winding streets and the town's "alphabet homes," single-family houses whose different models were known as "A," "B" and so on. The outside walls of the cheap, quickly built alphabet homes were made from a mix of asbestos and cement called "cemesto." Today they've been personalized, with a variety of colors and facade styles.
History and science buffs should visit the New Hope Center, which offers an exhibit devoted to the massive Y-12 atomic plant that separated the crucial uranium material, known as U-235, from ordinary uranium. After the "pretty blue-green powder" of the U-235 was carefully packaged, Wilcox recalled, it was slipped into a briefcase that was chained to the wrist of a lieutenant in civilian clothes.
Accompanied by armed escorts, the lieutenant took a train to New Mexico. There, scientists used the enriched uranium to make one of the bombs.