During the 1940s, the denizens of Oak Ridge, Tennessee, worked for the Manhattan Project, developing atomic weapons in their government-owned city. They went about their daily lives in the shadows of billboards exhorting them not only to support the war effort, but also to keep quiet about their jobs.
After Oak Ridge was acquired by the United States Army Corps of Engineers in 1942, it set up its very own graphics department. Many of the signs the department made were informational signs, safety warnings, and typical wartime propaganda. Others, however, addressed the particularly sensitive nature of the Manhattan Project's work, reminded workers to keep their lips sealed.
You can see more photos of Oak Ridge during World War II on the Department of Energy's Oak Ridge Flickr stream.