On this day in 1945, an atomic bomb on Hiroshima began the countdown to the end of World War II. On this day in 1945, an atomic bomb on Hiroshima began the countdown to the end of World War II. On this day in "Atomic City", Oak Ridge Tennessee, a group of young women learned what they had been doing all this time. ImageHere is one of the "Calutron Girls" at the Y-12 plant in Atomic City. The young women with no degrees were just as accurate but faster (and cheaper, and less temperamental) than the physicist so they were hired.