The light of a nuclear explosion is unlike anything else on Earth. This is because the heat of a nuclear explosion is unlike anything else on Earth. Seventy years ago today, when the first atomic weapon was tested, they called its light cosmic. Where else, except in the interiors of stars, do the temperatures reach into the tens of millions of degrees? It is that blistering radiation, released in a reaction that takes about a millionth of a second to complete, that makes the light so unearthly, that gives it the strength to burn through photographic paper and wound human eyes. The heat is such that the air around it becomes luminous and incandescent and then opaque; for a moment, the brightness hides itself. Then the air expands outward, shedding its energy at the speed of sound—the blast wave that destroys houses, hospitals, schools, cities.

The test was given the evocative code name of Trinity, although no one seems to know precisely why. One theory is that J. Robert Oppenheimer, the head of the U.S. government’s laboratory in Los Alamos, New Mexico, and the director of science for the Manhattan Project, which designed and built the bomb, chose the name as an allusion to the poetry of John Donne. Oppenheimer’s former mistress, Jean Tatlock, a student at the University of California, Berkeley, when he was a professor there, had introduced him to Donne’s work before she committed suicide, in early 1944. But Oppenheimer later claimed not to recall where the name came from.

The operation was designated as top secret, which was a problem, since the whole point was to create an explosion that could be heard for a hundred miles around and seen for two hundred. How to keep such a spectacle under wraps? Oppenheimer and his colleagues considered several sites, including a patch of desert around two hundred miles east of Los Angeles, an island eighty miles southwest of Santa Monica, and a series of sand bars ten miles off the Texas coast. Eventually, they chose a place much closer to home, near Alamogordo, New Mexico, on an Army Air Forces bombing range in a valley called the Jornada del Muerto (“Journey of the Dead Man,” an indication of its unforgiving landscape). Freshwater had to be driven in, seven hundred gallons at a time, from a town forty miles away. To wire the site for a telephone connection required laying four miles of cable. The most expensive single line item in the budget was for the construction of bomb-proof shelters, which would protect some of the more than two hundred and fifty observers of the test.

The area immediately around the bombing range was sparsely populated but not by any means barren. It was within two hundred miles of Albuquerque, Santa Fe, and El Paso. The nearest town of more than fifty people was fewer than thirty miles away, and the nearest occupied ranch was only twelve miles away—long distances for a person, but not for light or a radioactive cloud. (One of Trinity’s more unusual financial appropriations, later on, was for the acquisition of several dozen head of cattle that had had their hair discolored by the explosion.) The Army made preparations to impose martial law after the test if necessary, keeping a military force of a hundred and sixty men on hand to manage any evacuations. Photographic film, sensitive to radioactivity, was stowed in nearby towns, to provide “medical legal” evidence of contamination in the future. Seismographs in Tucson, Denver, and Chihuahua, Mexico, would reveal how far away the explosion could be detected.

The Trinity test weapon. Courtesy Los Alamos National Laboratory

On July 16, 1945, the planned date of the test, the weather was poor. Thunderstorms were moving through the area, raising the twin hazards of electricity and rain. The test weapon, known euphemistically as the gadget, was mounted inside a shack atop a hundred-foot steel tower. It was a Frankenstein’s monster of wires, screws, switches, high explosives, radioactive materials, and diagnostic devices, and was crude enough that it could be tripped by a passing storm. (This had already happened once, with a model of the bomb’s electrical system.) Rain, or even too many clouds, could cause other problems—a spontaneous radioactive thunderstorm after detonation, unpredictable magnifications of the blast wave off a layer of warm air. It was later calculated that, even without the possibility of mechanical or electrical failure, there was still more than a one-in-ten chance of the gadget failing to perform optimally.

The scientists were prepared to cancel the test and wait for better weather when, at five in the morning, conditions began to improve. At five-ten, they announced that the test was going forward. At five-twenty-five, a rocket near the tower was shot into the sky—the five-minute warning. Another went up at five-twenty-nine. Forty-five seconds before zero hour, a switch was thrown in the control bunker, starting an automated timer. Just before five-thirty, an electrical pulse ran the five and a half miles across the desert from the bunker to the tower, up into the firing unit of the bomb. Within a hundred millionths of a second, a series of thirty-two charges went off around the device’s core, compressing the sphere of plutonium inside from about the size of an orange to that of a lime. Then the gadget exploded.

General Thomas Farrell, the deputy commander of the Manhattan Project, was in the control bunker with Oppenheimer when the blast went off. “The whole country was lighted by a searing light with the intensity many times that of the midday sun,” he wrote immediately afterward. “It was golden, purple, violet, gray, and blue. It lighted every peak, crevasse, and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined. It was that beauty the great poets dream about but describe most poorly and inadequately.” Twenty-seven miles away from the tower, the Berkeley physicist and Nobel Prize winner Ernest O. Lawrence was stepping out of a car. “Just as I put my foot on the ground I was enveloped with a warm brilliant yellow white light—from darkness to brilliant sunshine in an instant,” he wrote. James Conant, the president of Harvard University, was watching from the V.I.P. viewing spot, ten miles from the tower. “The enormity of the light and its length quite stunned me,” he wrote. “The whole sky suddenly full of white light like the end of the world.”