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Trinity Site

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1944–1945, Leslie Groves, J. Robert Oppenheimer, Kenneth T. Bainbridge, U.S. Army Corps of Engineers. Junction of Wsmr P Route 20 and Wsmr P Route 13, White Sands Missile Range.
  • (Photograph by Christopher C. Mead)

On July 16, 1945, the world’s first nuclear fission bomb, code-named the Gadget, was detonated at Trinity Site in New Mexico. On August 6, a second atomic bomb was dropped on Hiroshima, Japan, and on August 9, a third atomic bomb was dropped on Nagasaki, Japan. Originally intended for use against Germany, the atomic bomb was retargeted on Japan in early 1945, after Germany’s defeat in World War II became inevitable. Whether its use was necessary to force Japan’s cessation of hostilities on August 15, 1945, remains a subject of historical debate.

The test at Trinity Site completed an accelerated process of research and production begun three years earlier when a secret wartime project, code-named the Manhattan Engineer District and soon shortened to the Manhattan Project, was established under the command of U.S. Army General Leslie Groves. The physicist J. Robert Oppenheimer was named director of the laboratory, designated Site Y of the Manhattan Project, where the world’s first atomic bombs would be designed and built.

Located in Los Alamos, New Mexico, the laboratory started operations in April 1943 and had, by March 1944, identified both a gun-type bomb using Uranium 235 and an implosion-type bomb using Plutonium 239. The parallel development of two bombs addressed a practical dilemma. With U-235, it was easier to control fission, yet the material was harder to produce. While a simple gun-type bomb would certainly work, by slamming together two quantities of U-235 along an intervening barrel, it would be difficult to build many of them. Conversely, P-239 was easier to produce, yet its higher radioactivity required a technically complex implosion bomb to prevent pre-detonation; an outer sphere of shaped explosive charges called lenses would force a subcritical spherical core of plutonium (inside a buffering “tamper” of uranium that increased the bomb’s efficiency) into a supercritical state by compressing it to less than half its original size. The unique gun-type bomb, code-named Little Boy, was dropped on Hiroshima without testing, but the implosion bomb required testing before the first of multiple copies, code-named Fat Man, could be dropped on Nagasaki.

In May 1944, the physicist Kenneth T. Bainbridge started scouting potential test sites that met the stipulated criteria for a flat, isolated area with good visibility and generally good weather. The site chosen was located two hundred miles south of Los Alamos, in a high desert basin between 4,000 and 5,000 feet, backed to the east and southeast against the Sierra Oscura and the San Andres Mountains, respectively. This desolate and semiarid region of scrubland was crossed by the Camino Real and had been known by the forbidding name of the Jornada del Muerto (Journey of the Dead Man) since circa 1670, when a German trader fleeing the Inquisition in the Province of New Mexico died traversing its waterless expanse. Large sections were owned by the state of New Mexico and leased to local ranchers and sheepherders. In January 1942, the federal government had acquired a piece of the Jornada for the Alamogordo Bombing and Gunnery Range, and the range commander agreed in November 1944 to set aside an 18 x 24-square-mile portion for use as an atomic test site.

Both the site and the test were code-named Trinity, probably by Oppenheimer. His reasons for this name remain unclear, though he reportedly was thinking of themes of death and destruction found in the poems of John Donne. The Army Corps of Engineers, assisted initially by the J. D. Leftwich Construction Company of Lubbock, Texas, and then by Ted Brown, a contractor from Albuquerque, New Mexico, built Trinity’s multiple structures, along with telephone and electrical lines and 40 miles of roads. A base camp called Camp Trinity was set up with tents, barracks, mess hall, kitchen, a commissary and other buildings at the ranch of David McDonald ten miles south of the test site. The ranch of George McDonald, two miles southeast of the test site, was adapted to serve as a field laboratory and bomb assembly station.

At Ground Zero, a 100-foot tower was erected from prefabricated steel sections with four crossed-braced stanchions set on reinforced concrete footings dug 20 feet deep and spaced 35 feet apart. The tower was surrounded by a radiating series of bunkers with concrete slab roofs on massive oak timbers and protective earth embankments. These bunkers were identified by their orientation and distance in yards from Ground Zero: four unmanned instrument bunkers at North 800, North 1000, West 500, and West 800, and three observation bunkers, one at North 10,000 with instruments and searchlights, one at West 10,000 with searchlights and high-speed cameras, and the control bunker at South 10,000. Twenty miles northwest of Ground Zero, an observation point for visiting dignitaries was prepared with trenches atop Compañia Hill.

A second tower, built with wooden trestles stacked 20 feet high and located 800 yards south of Ground Zero, was loaded with 100 tons of TNT and detonated on May 7, 1945. The explosion was used to calibrate instruments, and to gauge the potential spread of radiation (small amounts of radioactive materials were inserted in tubes run through the TNT).

The test date for the nuclear explosion itself was finally set for July 16, after delays in casting explosive lenses for the Gadget postponed the original test date of July 4. Political considerations overrode the threat of summer thunderstorms in fixing this date. The test was timed to coincide with the Potsdam Conference (itself rescheduled to July 17–August 2 after the test on July 4 was cancelled) and thus strengthen the hand of President Truman in his negotiations with Joseph Stalin over the post-war world order.

The bomb components were delivered from Los Alamos and assembled at the George McDonald ranch house on July 12 and 13. On the morning of July 14, the Gadget was hoisted into a sheet metal shed at the top of the 100-foot tower, and its 32 detonators were installed. Oppenheimer’s brother, Frank, set out pine boxes filled with wood shavings, and posts nailed with strips of corrugated iron, at distances of up to 2,000 yards from Ground Zero, in a last-minute, impromptu experiment designed to model the effects of such a bomb on the wooden houses of Japan. Final wiring of the Gadget’s detonators was carried out during the night of July 15, but continuing rain and lighting delayed the scheduled 4:00 AM detonation on July 16. After a favorable weather report forty-five minutes later, the final countdown began at 5:10 AM. In the control bunker—South 10,000—the last of a set of switches was thrown at T-minus-45-seconds, initiating the detonation mechanism that triggered the Gadget at 5:29:45 AM.

Space became a factor of time as an instant chain reaction of nuclear fission generated a blast point several million degrees Celsius, expending approximately 18 percent of the plutonium core before heat turned the bomb into an ionized gas and stopped the chain reaction. A superheated isothermal sphere formed at one millisecond after detonation. This released a brilliant flash of light (actually two flashes milliseconds apart) that lit up the sky like multiple suns, and fueled a fireball with an expanding shock wave of radiation, heat, and pressure. At 32 milliseconds after detonation, the fireball reached a diameter of 945 feet, and by 0.85 seconds it had reached a diameter of 2,500 feet.

The blast was equivalent to approximately 18.6 kilotons of TNT. It vaporized the steel tower at Ground Zero and made a crater 8-10 feet deep at its center and a half-mile across, fusing the sandy soil into a radioactive green glass called trinitite. Frank Oppenheimer’s pine boxes were charred at 1,000 yards and scorched at 2,000 yards, and every living thing within a mile of Ground Zero—plants, ants, lizards, animals—was killed. An enormous cloud of dust enveloped the fireball, which began to surge upward into a luminescent mushroom cloud that, seven minutes later, had climbed to 38,000 feet. Spreading out and splitting into parts that drifted to the north and west, this cloud began dropping radioactive fallout several hours later over an area 30 miles wide and 100 miles long.

Oppenheimer claimed that a line from Hindu scripture, the Bhagavad Gita, flashed through his mind at the moment of detonation: “Now I am become Death, the destroyer of worlds.” The atomic age had dawned, with consequences the world has tried to control ever since.

First opened to the public in 1953, Trinity Site can be visited twice a year on the first Saturday in April and October. A duplicate casing of the Fat-Man bomb dropped on Nagasaki, Japan, is trucked onto the site for this annual open house.


DeGroot, Gerald J. The Bomb: A Life. Cambridge, MA: Harvard University Press, 2005.

Greenwood, Richard, “Trinity Site,” Socorro County, New Mexico. National Register of Historic Places Registration Form, 1975. National Park Service, U.S. Department of the Interior, Washington, D.C.

Merlan, Thomas, Michael P. Marshall, John Roney. “Camino Real in New Mexico, AD 1598–1881,” Socorro County, New Mexico. National Register of Historic Places Registration Form, 2010. National Park Service, U.S. Department of the Interior, Washington, D.C.

Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon and Schuster, 1986.

Seeber, Robert. The Los Alamos Primer: The First Lectures on How to Build An Atomic Bomb. Edited and introduction by Richard Rhodes. Berkeley and Los Angeles: University of California, 1992.

Szasz, Ferenc Morton. The Day the Sun Rose Twice: The Story of the Trinity Site Nuclear Explosion July 16, 1945. Albuquerque: University of New Mexico, 1984.

Writing Credits

Christopher C. Mead
Christopher C. Mead
Regina N. Emmer



  • 1944

    Built and destroyed
  • 1965

    Designated a National Historic Landmark

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Christopher C. Mead, "Trinity Site", [Socorro, New Mexico], SAH Archipedia, eds. Gabrielle Esperdy and Karen Kingsley, Charlottesville: UVaP, 2012—,

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