In 1939 a group of scientists, Albert Einstein amongst them, warned President Roosevelt of the possibility that Hitler’s Germany might be close to producing an atomic bomb. Roosevelt issued an order to the effect that whatever it involved, the US had to be the first to develop the atomic bomb.

Within three years they had started to create a hidden world of secret cities and classified nuclear facilities, built deep inside America. Four-hundred thousand people were involved in the project, although 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.

Three top secret cities were built in isolated parts of Tennessee, New Mexico and Washington State, requiring half a million acres of land, billions of dollars and the construction of the largest building in the world. This was to be the most costly and labour intensive engineering programmes ever undertaken and became known as the Manhattan Project.