The world’s first nuclear weapons test took place on July 16, 1945 in the desolate White Sands deserts of New Mexico. In a cryptic reference to a John Donne poem that he knew and loved, J. Robert Oppenheimer, lead physicist of the Manhattan Project and scientific director of the test, dubbed the location “Trinity.”

At 5:29:45 a.m. local time, a plutonium-based atomic bomb was detonated atop a 100-foot steel tower erected at Trinity specifically for the test. Scientists hoped that exploding the bomb at an elevated height would reduce the amount of radioactive dust raised by the explosion. They also needed to simulate the air-drop method of deployment that was eventually used by the real bombs.

The Trinity bomb was an exact replica of “Fat Man,” the second and last nuclear weapon ever used in war. Fat Man was detonated over Nagasaki, Japan less than a month after the Trinity test.

Exploding with an energy equal to about 20 kilotons of TNT, the blast carved a crater in the Earth more than 1,000 feet wide and 10 feet deep. Radioactive fallout from the blast was detected as far away as Indiana.

Heat from the explosion was so intense that sand grains fused to form a reflective layer of radioactive, green glass, called “Trinitite,” on the desert floor.