The Manhattan Project: How 125,000 People Built the Atomic Bomb in Secret

A Letter That Changed Everything

On August 2, 1939, Albert Einstein signed a letter to President Franklin Roosevelt warning that Germany might develop an atomic bomb. The letter, drafted by physicist Leo Szilard, described the theoretical possibility of nuclear chain reactions in uranium. Roosevelt responded by creating the Advisory Committee on Uranium, which evolved into the Manhattan Engineer District — code name for the largest secret scientific project in history. Over six years, the project consumed $2 billion (approximately $30 billion in 2024 dollars), employed 125,000 people at 30 sites across the United States, Canada, and the United Kingdom, and produced three nuclear devices.

Most workers had no idea what they were building. Compartmentalization was absolute.

The Scientific Foundation

Nuclear fission — the splitting of heavy atomic nuclei — was discovered in December 1938 by Otto Hahn and Fritz Strassmann in Berlin. Lise Meitner and Otto Frisch provided the theoretical explanation weeks later. The physics was straightforward: splitting a uranium-235 atom releases energy and additional neutrons, which can split more atoms in a self-sustaining chain reaction. The engineering challenge was immense.

Two Paths to a Bomb

• Uranium bomb (gun-type) — Fire one subcritical mass of uranium-235 into another to achieve supercriticality. Conceptually simple but required separating U-235 (0.7 percent of natural uranium) from U-238. This became the Little Boy design.
• Plutonium bomb (implosion-type) — Compress a subcritical sphere of plutonium-239 using precisely shaped explosive lenses to achieve supercriticality. More efficient but far more complex. This became the Fat Man design.

Secret Cities and Industrial Scale

The Manhattan Project built three secret cities from scratch, each dedicated to a different phase of weapons production.

Oak Ridge consumed more electricity than New York City at peak operation. The K-25 gaseous diffusion plant covered 44 acres — the largest building in the world at the time. Hanford's B Reactor was the world's first full-scale nuclear reactor. These facilities represented engineering achievements on a scale never previously attempted for a single project.

The Scientists of Los Alamos

J. Robert Oppenheimer directed the Los Alamos laboratory, assembling the greatest concentration of scientific talent in history. The roster included twelve present or future Nobel laureates. Enrico Fermi had demonstrated the first controlled nuclear chain reaction at the University of Chicago on December 2, 1942. Niels Bohr consulted on reactor design. Hans Bethe led the theoretical division. Richard Feynman, then 24, worked on implosion calculations.

The implosion design posed the greatest technical challenge. Conventional explosives had to compress a plutonium core symmetrically to within fractions of a millimeter. Asymmetric compression would produce a fizzle rather than a detonation. Seth Neddermeyer proposed the concept, and George Kistiakowsky led the explosive lens development team that solved the precision problem.

Trinity: The First Nuclear Detonation

At 5:29 a.m. on July 16, 1945, the first nuclear device detonated at the Trinity test site in the Jornada del Muerto desert, New Mexico. The explosion yielded approximately 21 kilotons of TNT equivalent — vastly exceeding the scientists' median prediction of 5 kilotons. The fireball reached a temperature of 10 million degrees Fahrenheit. The blast created a crater 5 feet deep and 30 feet wide, fusing the desert sand into a glassy substance later named trinitite.

Oppenheimer later recalled that the Hindu scripture Bhagavad Gita crossed his mind: "Now I am become Death, the destroyer of worlds." Test director Kenneth Bainbridge was more direct: "Now we are all sons of bitches."

Hiroshima and Nagasaki

On August 6, 1945, the B-29 bomber Enola Gay dropped Little Boy on Hiroshima. The uranium gun-type bomb detonated at 1,900 feet with a yield of approximately 15 kilotons. Between 70,000 and 80,000 people died instantly. By the end of 1945, the death toll reached an estimated 140,000 from blast injuries, burns, and radiation sickness.

Three days later, on August 9, Bockscar dropped Fat Man on Nagasaki. The plutonium implosion bomb yielded approximately 21 kilotons, killing an estimated 40,000 people immediately and 70,000 by year's end. Japan announced its surrender on August 15, 1945.

• Military rationale — Proponents argued the bombings prevented a land invasion of Japan that could have cost hundreds of thousands of American and millions of Japanese lives.
• Strategic critique — Historians like Gar Alperovitz argued that Japan was already near surrender and the bombings were partly motivated by demonstrating power to the Soviet Union.
• Ethical debate — The deliberate targeting of civilian populations raised moral questions that persist to this day. Many Manhattan Project scientists, including Szilard, petitioned against using the bomb on populated cities.

Legacy and the Nuclear Age

The Manhattan Project initiated the nuclear arms race. The Soviet Union detonated its first atomic bomb in 1949, aided partly by espionage from Los Alamos physicist Klaus Fuchs. By 1986, global nuclear arsenals peaked at approximately 64,000 warheads. The project also launched civilian nuclear power, nuclear medicine, and the model of large-scale government-funded scientific research that produced the space program, the internet, and the Human Genome Project.

The moral weight of the Manhattan Project defined the relationship between science and society for the rest of the twentieth century. Oppenheimer himself was stripped of his security clearance in 1954 during anti-communist hearings — a decision the U.S. government formally vacated in 2022. The project demonstrated both the extraordinary capability and the profound responsibility that accompanies scientific discovery at the frontier of human knowledge.