Beginning in 1939, some American scientists–many of them refugees of fascist regimes in Europe–advocated the development of ways to use nuclear fission for military purposes. By late 1941, the federal government’s Office of Scientific Research and Development, headed by scientist Vannavar Bush, took control of the project. After the United States entered World War II, the U.S. Army Corps of Engineers was tasked with building the great quantity of necessary plants, laboratories and other research and testing facilities.

Much of the initial research had been performed at Columbia University in New York City, and the top-secret research was thereafter known by the code name Manhattan Project. More than 30 laboratories and sites and more than 130,000 people were eventually involved in different facets of nuclear research and development, with three primary locations–in Oak Ridge, Tennessee; Richland, Washington; and Los Alamos, New Mexico–that became virtual top-secret atomic cities.

A medium-sized reactor built at Oak Ridge produced uranium-235 and plutonium, both of which would be used as vital components in the atomic bomb. The Oak Ridge facility produced the majority of uranium used to build the “Little Boy” bomb that would be dropped over the Japanese city of Hiroshima in August 1945.

Within a year, the world’s first large-scale plutonium reactor was in service at Hanford, and by early 1945 shipments of enriched plutonium from the plant’s three reactors were being sent to Los Alamos every five days. This material would be used in the first atomic bomb testing, as well as in “Fat Man,” the atomic bomb dropped over Nagasaki.

Finally, The facility at Los Alamos served as the primary “think tank” of the Manhattan Project. Its engineers, led by J. Robert Oppenheimer, were responsible for the final construction, testing and delivery of the bombs.

At 5:30 a.m. on July 16, 1945, Los Alamos scientists detonated a plutonium bomb at a test site located on the U.S. Air Force base at Alamogordo, New Mexico, some 120 miles south of Albuquerque. Oppenheimer chose the name “Trinity” for the test site, inspired by the poetry of John Donne. The test had been scheduled for 4 a.m., but when the time came it was raining, and the appointed hour was pushed back to 5:30.

Tensions ran high at the test site, where those assembled included the scientist Enrico Fermi–who had directed the first nuclear chain reaction in December 1942–U.S. Army Brigadier General Leslie Groves, Bush, Oppenheimer and others.

When the bomb was finally detonated atop a steel tower, an intense light flash and sudden wave of heat was followed by a great burst of sound echoing in the valley. A ball of fire tore up into the sky and then was surrounded by a giant mushroom cloud stretching some 40,000 feet across. With a power equivalent to around 21,000 tons of TNT, the bomb completely obliterated the steel tower on which it rested. The nuclear age had begun.