The night of the 5th to the 6th of August 1945 had been a night of

of sirens in the Japanese city of Hiroshima, but by 7 a.m. the all clear had been sounded.

Around an hour after those sirens had quieted, the American B-29 bomber,

the Enola Gay, began its bomb run over the city. And at 815, the aircraft's

bombardier released one bomb from an altitude of 31,000 feet. In the months previous, Japan had been utterly incinerated by tons of bombs dropped in huge numbers by vast fleets of American bombers.

On this day, though, on this 6th of August, a single bomb would do the damage

of thousands of conventional bombs because this one was an atomic weapon.

Inside the casing of that bomb was not high explosives, but 64

kilograms of Uranium 235. It took less than a minute for that bomb to travel

from 31,000 feet to 600 meters above the city streets, where it

detonated. A nuclear reaction then took place. Neutrons crashed into the nuclei of uranium

atoms, causing a fission chain reaction, more neutrons

splitting more and more atoms. Unimagitable energy was released, and with it enormous destruction. The largest bomb

ever dropped in history to that point had had the blast equivalent of 6.5 tons

of TNT. This bomb had an explosive force equivalent to 15,000 tons of TNT.

On the ground, survivors remember seeing a brilliant flash of light, then a very loud boom.

In a split second, an area with a diameter of two miles was completely destroyed.

Buildings were leveled, a few bits of twisted superstructure remained.

And after that initial blast, there came the firestorm. Hiroshima's paper and wooden houses were consumed in a blaze that sent giant clouds soaring up into the atmosphere.

Around one in three people in the city were killed, 70 to 80,000 people, it's impossible to be certain.

They were killed by the initial explosion, or they were killed in the firestorm, or they were killed by radiation poisoning in the weeks and months

had followed.

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