Top Photo: Mushroom cloud of 'Gadget' over Trinity, seconds after detonation. July 16, 1945. United States Department of Energy

In the 1930s, physicists explored the possibility of splitting the atom to create an "atomic bomb." Scientists in Nazi Germany made a major breakthrough in 1938 when they split the uranium atom. Fearing what Hitler would do if he had such a devastating new weapon, the United States undertook the "Manhattan Project," the largest, most secretive wartime project in history. The program coordinated the work of scientists, engineers, and factory workers at sites across America. Few were aware of the project's end goal — to develop, test, and deliver an atomic weapon —and no one knew if the unprecedented effort could succeed.

The Race for the Weapon

After Hitler came to power, leading European scientists, many of them Jewish, fled Nazi persecution. Some came to the United States, bringing knowledge of German atomic research and fears of the destruction that could be unleashed. In 1939, refugee physicists Albert Einstein and Leo Szilard drafted a letter to President Roosevelt, describing the potential for a nuclear chain reaction bomb. Even though the bomb was only theoretically possible, Roosevelt heeded their warning and launched what would become the top-secret Manhattan Project. Scientists from around the world joined, racing to beat Hitler to the bomb.

Innovation Great and Terrible

Amidst fears of Germany's atomic research, a British report in July 1941 was encouraging. British scientists argued that producing an atomic bomb was feasible within two years. Two months before Pearl Harbor, Roosevelt authorized extensive research to determine whether the US could build a bomb, and in January 1942, he approved atomic weapons production. Even as the experts struggled to design the key ingredients, construction had already begun on gigantic facilities. To overtake Germany in the race for a bomb, Manhattan Project leaders pursued four different processes for producing both plutonium and uranium, gambling that at least one of them would work.

The quest to build an atomic bomb before Nazi Germany did was the culmination of an epic scientific journey. It had only been 40 years since Einstein proved the existence of the atom. Trying to split one posed a daunting challenge, requiring huge new facilities, a large but discreet workforce, new safety protocols, and a vast supply of electrical power. Above all, the Manhattan Project was a gamble, spending billions of dollars to try to turn atomic theory into physical reality. The Manhattan Project pursued the ultimate weapon for the Arsenal of Democracy, but no one knew if it would succeed.

The Secret Cities

In late 1942, the Army Corps of Engineers selected three remote sites for major Manhattan Project research and production facilities. Forbidden by the government to say where they lived or what they did, thousands of atomic workers lived and worked in "secret cities" hastily constructed behind barbed-wire fences. In Los Alamos, New Mexico, P.O. Box 1663 served as the mailing address for all residents. In Hanford, Washington, the government monitored phone calls to protect against security breaches. The enormous complex at Oak Ridge, Tennessee, did not even appear on maps.

The Trinity Test

By June 1945, Los Alamos had built a test device using plutonium. Oppenheimer was unsure of the power of "the Gadget," as he called it, wagering the explosion would equal 300 tons of TNT. On July 16, 1945, the Atomic Age began with a burst of blinding light, searing heat, and an echoing roar, as a mushroom cloud rose above the desert. The explosion packed the power of 15,000 tons of TNT - 50 times Oppenheimer's prediction. Trinity was a monumental scientific achievement, but a critical question remained: with Germany defeated, could this weapon end the escalating war against Japan?