'Trinity' was the US operational test of the world’s first atomic device at Alamogordo, New Mexico (16 July 1945).

The test was undertaken by the US Army in the Jornada del Muerto desert about 35 miles (56 km) to the south-east of Socorro, New Mexico, at the new White Sands Proving Ground, which incorporated the Alamogordo Bombing and Gunnery Range.

'Trinity' was a test of an implosion-design plutonium device, nicknamed 'The Gadget', and the detonation produced an explosive power equivalent to the explosion of about 20 kilotons of TNT. There was a pre-test explosion of 108 tons of TNT, spiked with fission products from the Hanford reactor, on 7 May to calibrate the instruments.

For the full 'Trinity' test, the device was hoisted to the top of a 100-ft (30-m) steel tower for detonation as the height was deemed to offer a better indication of how the weapon would behave when dropped from an aeroplane: it was believed that detonation in the air would maximise the amount of energy applied directly to the target as the detonation expanded in a spherical shape, and as it would generate less nuclear fall-out.

The device was assembled by Rudolf Peierls at the nearby McDonald Ranch House on 13 July, the day after the components had arrived. The device was winched up the tower on the following day. Major General Leslie R. Groves, heading the 'Manhattan Project', had ordered the construction of a 214-ton steel canister code-named 'Jumbo' to recover valuable plutonium if the five tons of conventional explosives failed to compress the plutonium into a chain reaction. The container was constructed in Pittsburgh, Pennsylvania, and brought to the test site by rail, but by the time it arrived, the confidence of the scientists was high enough that they decided not to use it. Instead, it was hoisted up in a steel tower 800 yards (730 m) from the gadget as a rough measure of how powerful the explosion would be. In the end, 'Jumbo' survived, though its tower did not.

The detonation was scheduled for 04.00 but was then postponed because of rain and lightning earlier that morning: it was feared that the danger from radiation and fall-out would be greatly increased by rain, and lightning had the scientists concerned about accidental detonation. At 04.45 a crucial weather report was favourable and, at 05.10 the 20-minute countdown began. Most top-level scientists and military officers were observing from a base camp 10 miles (16 km) to the south-west of the test tower. Many other observers were about 20 miles (32 km) away, and some others were scattered at different distances, some in more informal situations.

At 05.29.21 the device was detonated, resulting in the creation in the desert of a crater of radioactive glass 10 ft (3 m) deep and 1,100 ft (335 m) in diameter. It took 40 seconds for the shock wave to reach the observers, and was felt to a radius of 100 miles (160 km). The mushroom cloud reached an altitude of 40,000 ft (12190 m).