This is what happened the morning the first atomic bomb created a new world

Fifteen seconds before 5.30 am on July 16 1945, above an area of New Mexico desert so unforgivingly dry that earlier travellers christened it the Jornada del Muerto (Journey of the Dead Man), a new sun flashed into existence and rose rapidly into the sky. It was a little before dawn.

This strange, early daybreak was the Trinity Test: humanity’s first encounter with the atomic bomb. Within a month two bombs were dropped on Japan: the first, “Little Boy”, a uranium weapon, at Hiroshima; the second, “Fat Man”, a plutonium weapon of the implosion design tested at Trinity, on Nagasaki. Casualty estimates vary widely, but perhaps as many as 150-250,000 people died as a direct result of these two events. The following half century was one of intense nuclear testing, the residue of which might be the signature for the proposed new epoch of the Anthropocene.

The extraordinary story of the Manhattan Project, which led to this point, has been told many times. It begins with the realisation that atomic weapons, releasing vast amounts of energy via a nuclear chain reaction, were possible. It includes a 1939 letter, signed by Albert Einstein, alerting President Roosevelt to the dangers of a German atomic bomb programme, and tells how, following the United States’ entry into the second world war after the Japanese attack on Pearl Harbor, the programme accelerated rapidly under the control of General Leslie Groves.

The Manhattan Project absorbed the British and Canadian “Tube Alloys” atomic programme, and drew on a dazzling array of scientific talent. More than a purely scientific endeavour, it was an engineering and industrial enterprise on a massive scale, employing about 130,000 people at its peak, and perhaps half a million cumulatively.

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Site Y was a town built from scratch to build the atomic bomb at Los Alamos, New Mexico. Here, under the scientific directorship of J Robert Oppenheimer – a complex, charismatic figure (so famous after the war that he was instantly recognisable by his porkpie hat) – scientists, including many who’d fled Nazi persecution in Europe and were acutely aware of what a Nazi bomb might mean, built the “gadget” tested at Trinity.

By then, though, circumstances had changed. In late 1944, as Allied forces advanced across Europe, it became apparent that the German bomb programme had stalled years before. After Franklin Roosevelt’s death in April 1945 and Germany’s defeat in May, the Trinity Test was prioritised so Harry Truman, the new president, would have news of it when he met Joseph Stalin and Winston Churchill at the Potsdam conference.

Trinity is a striking moment. Scientists, military personnel and observers gathered in observation bunkers 10,000 yards from ground zero, at a base camp ten miles away, and at Compañia Hill, 20 miles away. Overnight, thunder, lightning and rain swept across the area, imperilling the test.

Don Hornig, the last man to “babysit” the bomb in its metal shack at the top of a 100ft tower, recalls passing the time by reading an anthology of humorous writing, Desert Island Decameron, by the light of a 60-watt bulb. He hoped the wet tower would act as a lightning rod if there were any lightning strikes. The alternative was sobering, but he appears to have been philosophical: “It would set the bomb off. And in that case, I’d never know about it! So I read my book.”