S1, E5: A team of field investigators uncovers the clues that will recreate vanished or hidden worlds. They use the latest research, expert analysis and cutting edge graphic technology to take us back. In 1939 a group of scientists -- Albert Einstein among them - warned President Roosevelt, of the possibility that Hitler's Germany might be close to producing an atomic bomb. Roosevelt issued an order -- whatever it takes, the US has to be the first to develop an atomic bomb. And within three years they were well on their way to creating a hidden world of secret cities and classified nuclear facilities, built inside America. 400,000 people were part of it -- though only a handful really knew the truth. Six decades later, a team of experts return to the once-classified sites where the course of history was decided. In top secret cities and nuclear facilities, they uncover and rebuild this lost world. Requiring half a million acres of land, billions of dollars and the construction of the largest building in the world, three top secret cities were built in isolated parts of Tennessee, New Mexico and Washington State. This was to be the most costly and most labor intensive engineering program ever undertaken. Using eyewitness testimony and cutting edge graphic technology, we recreate the secret world of the Manhattan Project today.