Three atomic bombs were ready by the end of June 1945 as the United States planned what promised to be a very tough and bloody invasion of imperial Japan.

Two of the bombs -- nicknamed "Little Boy," a uranium device; and "Fat Man," a plutonium bomb -- were readied for shipment to the Pacific in the hope of ending World War II and avoiding a US massacre on Japanese beachheads.

But while the scientific team was confident the uranium bomb would work, the plutonium device had to be tested as quietly as possible before it was dropped on Japan.

The most powerful weapon in history, dubbed the "gadget" by its designers, was hoisted onto a 100-foot (30-meter) steel tower at the new Trinity Site after US president Harry Truman gave the go-ahead.

All that was left was for scientists to electronically trigger the detonation of the bomb, identical to the one that would be dropped on Nagasaki, from their bunker 10 kilometers (six miles) away.

A few seconds before 5:30 am on July 16, the countdown ended and a massive conventional explosion took place, sparking a nuclear reaction in the weapon's plutonium core and turning the night sky to day.

After sunrise, a team armed with gear to protect them from radiation made their way to the blast site, from which a huge circle of scorched earth had radiated.

The tower there had completely vanished, apart from two steel footings set in concrete, and a shallow 800-meter (800-yard) crater that was coated in green glass had appeared where the heat had melted the desert sand.

Just four hours after the test, the Navy cruiser "USS Indianapolis" set sail from San Francisco towards the Pacific island of Tinian with a secret cargo on board: "Little Boy," which would wreak death and destruction on the Japanese city of Hiroshima on August 6.

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