When the Manhattan Project was founded in 1941, General Leslie R. Groves was put in charge and tasked with finding locations for the top-secret project.
The three main locations were Hanford, WA, Los Alamos, NM, and an area of farmland in East Tennessee which was later called Oak Ridge.
But during the war, it had no official name, appeared on no map and was ringed with chain link and security guards. Here in this "Secret City," over 75,000 workers toiled night and day at tasks which even they did not understand. It was not until the first atomic bomb was dropped on Hiroshima, Japan, that their work was revealed - Oak Ridge's task had been refining uranium ore into fissionable material. Today, Oak Ridge is the home to the Y-12 National Security Complex and the Oak Ridge National Laboratory - two of the top scientific establishments in the world.
The sign is simple to understand.