The McDonald Ranch House in the Oscura Mountains of Socorro County, New Mexico, was the location of assembly of the world's first nuclear weapon.

The active components of the Trinity test "gadget", a plutonium Fat Man-type bomb similar to that later dropped on Nagasaki, Japan, were assembled there on July 13, 1945.

The ranch was vacated by the McDonald family under protest in 1942, when the United States Army took over the land as part of the Alamogordo Bombing and Gunnery Range to use in training bomber crews during World War II.

[4] An addition was constructed on the north side by the McDonald family,[5] who had moved into the area in the late 1870s or early 1880s, and acquired the ranch house in the 1930s.

An ice house is located on the west side, along with an underground cistern which stored rain water running off the roof.

At one time, the north addition contained a toilet and bathtub, which drained into a septic tank northwest of the house.

[5] The ranch was vacated by the McDonald family under protest in 1942,[6] when the Alamogordo Bombing and Gunnery Range took over the land to use in training bomber crews during World War II.

[5] The plutonium hemispheres for the pit of the Trinity nuclear test "gadget" (bomb) were delivered to the McDonald Ranch House on July 11, 1945.

Brigadier General Thomas F. Farrell, the deputy director of the Manhattan Project, signed for them, and handed them over to Louis Slotin, the head of the Pit Assembly Team.

[9] The explosion only 2 miles (3.2 km) away blew most of the home's windows out but did not significantly damage the structure; years of rain water intrusion through the roof were responsible for decades of subsequent deterioration.

[4] Shortly after, Fulwyler acquired funding from the Department of Energy and the Army for the National Park Service to completely restore the house to the way it appeared on July 12, 1945.

[5] In 2014, the White Sands Missile Range announced that due to budgetary constraints, the site would only be open once a year, on the first Saturday in April.