Below, thousands of people were instantly carbonised in a blast that was thousands of times hotter than the sun's surface; further from the epicentre, birds ignited in mid-flight, eyeballs popped and internal organs were sucked from bodies of victims.
By the end of the day an estimated 160,000 were dead or injured and the bomb's "ghosts" walked the city - thousands of initial survivors who would die within days, often with the word mizu -water - on their lips. Many more subsequently died - and are still dying - from various cancers.
Harry Truman, the then President of the Unites States who had ordered Hiroshima destroyed, later said: "We have discovered the most terrible weapon in the history of the world," but steadfastly defended its use and said it had ultimately saved lives.
In March this year, Paul Tibbets, the pilot of the Enola Gay, also said the bomb had saved lives. Asked whether he had any regrets, he said: "Hell no, no second thoughts. If you give me the same circumstances, hell yeah, I'd do it again."
J Robert Oppenheimer, the brilliant scientist who oversaw the building of the bomb, was more ambiguous about his creation. He famously said after the first test detonation: "Now I am become death, the destroyer of worlds."
Truman's successor, President Dwight Eisenhower, also had reservations. In a 1963 interview with Newsweek magazine, he said: "The Japanese were ready to surrender and it wasn't necessary to hit them with that awful thing."
Some thought that Imperial Japan, like Nazi Germany, deserved what it got for the brutal, relentless bombing of Shanghai and Chongqing, the Rape of Nanjing and other war atrocities across Asia. But others asked where had the moral high ground of the Allies gone since President Franklin D Roosevelt described the 1940 Nazi blitzkrieg of British cities as "inhuman barbarism"?
"No one seemed conscious of the irony," wrote the US historian Howard Zinn. "One of the reasons for the general indignation against the fascist powers was their history of indiscriminate bombing of civilian populations."
Tomorrow morning, Hiroshima will sidestep the endless debates over whether the bombing was justified and concentrate on commemorating the victims, in a ceremony swelled by thousands of foreign visitors and dominated by fresh concerns that the world is forgetting the lessons learnt here.
"I'm less resentful about what happened in Hiroshima than I am about America's wars today," says Kazuko Kojima, who was born two days after the bomb fell, in a cellar filled with the dead and dying victims. "Why don't they stop? Aren't there better ways to solve problems? The reason people go to war is because they don't understand the feelings of others."
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