The atomic bomb detonated in the New Mexico desert at 05:29:45 local time on 16 July, 1945, "lit up the entire world". That is how Private Daniel Yearout, one of the few remaining eyewitnesses some 60 years on, recalls the morning the powers of the atom were first unleashed.
By Kathryn Westcott
Asked for his first thought after the test, top scientist J Robert Oppenheimer quoted from his favourite Hindu poem, The Bhagavad-Gita: "I am become Death, the destroyer of worlds."
One eyewitness said it was as if someone had turned the sun on with a switch. Picture: Los Alamos National Laboratory
Oppenheimer and other world-leading scientists who had taken part in the top-secret test knew that from that moment on, the world had changed forever.
For others who were involved, such as Private Yearout, it would be some time before they fully realised what had taken place.
The world would not know the full secret until 6 August, when the Japanese town of Hiroshima was bombed.
Daniel Yearout, a 25-year-old army private with the US Corps of Engineers, was deployed close to what became known as "Ground Zero" on that morning in July.
In 1945, Private Yearout was based at Los Alamos, the secret town that the US government had built during the war in the remote hills of New Mexico. It was here that a laboratory was established to design a nuclear weapon that the army hoped would win World War II.
Some 8,000 people lived and worked in the town - scientists and their families, engineers, technicians, secretaries and army personnel. They had more or less disappeared from the world and set up their own communities. While each played their part, few fully understood the magnitude of the work that went on there.
"The Los Alamos project was the best secret there's ever been," says Mr Yearout, who now lives in Waverly, Tennessee.
Deployed in the desert
On Saturday 14 July, 1945, Private Yearout and other members of the US Corps of Engineers left their base to take part in a "top-secret mission".
"No one knew where we were going or what was going to happen," Mr Yearout told the BBC News Website.
The officers were given telephone numbers to call along the way to find out where to go to next.
The convoy travelled some 200 miles into the desert to a place called Alamorgordo, about 18 miles from "Ground Zero".
They had been stationed in case the small communities in the probable fallout path needed to be evacuated.
"We were called out the night before. One of the officers told us we were going to take part in some testing," says Mr Yearout. "He said that if everything went well, the war would be over in a few days. But, then he said that if it all went wrong, 'it was each damn man for himself'."
Early on Monday morning Private Yearout and a few of his colleagues climbed a hill. They had been told it would be the safest place to be.
The day of the actual test began with an early morning thunderstorm. "There was someone running a camera up on the hill. We lay there and talked to him for a bit. The test was supposed to take place at around 0400 but was delayed because of the weather."
Shortly before 0530, half an hour before sunrise, the scientists went to bunkers six miles from the test site and put on sunglasses and sunscreen. The test began and the sky was lit up by an unnatural ball of fire.
"I don't remember whether I was standing up or lying against the fence," says Mr Yearout.
"Suddenly, without any sound, the whole world lit up. When I came to my senses, I was lying on the ground with my back to where the light was coming from. I put my hands over my eyes to protect them and I could see the bones in my fingers. It was as if I was looking at an X-ray.
"I whisked around and looked towards the light. I could hear a rumble and the Earth shook. I saw a big fireball rising in the sky - it looked like it was pouring gasoline out there, all the way around. The fireball was getting bigger and bigger and we just stood and watched.
"This was followed by a long rumbling - I'd say it went on for 10 minutes. In and out and round the mountains. The fire began going down and then I saw a swirl of black smoke rising in the sky.
"I was scared at the time. I didn't know what was going on. I remember the man running the camera beside us hollering that it was the most beautiful picture he had ever taken in his life - he said it maybe 25 times. All he was interested in was the picture and all I was wondering was if we were going to get out of there or not."
Eventually the men went back down the hill to their tents and started a game of poker. Radiation readings stayed at what was then considered safe levels and no one needed to be evacuated.
"No one was allowed to talk about what we saw," says Mr Yearout. "Anyone who did was shipped out pretty quickly."
The flash released four times the heat of the interior of the sun and was seen 250 miles away. But, so secret was the mission - codenamed Trinity - that local media were told that an ammunition dump had blown up on an army base in the area.
The test was code-named Trinity, supposedly after a poem by John Donne which begins: "Batter my heart, three-person'd God"
In Potsdam outside Berlin, President Harry Truman waited for the coded message that the bomb had been successful.
Mr Yearout says Trinity paved the way for bringing an end to the war and saving many American and Japanese lives. "If we had gone into Japan, we would have encountered the worst fighting we ever had ever seen. We would have been there for four to six years."
But 60 years on, debate still rages over whether the bomb was really necessary to force the Japanese to surrender.
A number of the scientists involved in the project ended up feeling extremely ambivalent about the bomb's use, and some went on to campaign against nuclear arms.
For Mr Yearout, Trinity remains one of the 20th Century's most significant achievements.
"I was glad I'd seen it," he says. "But I hope I don't see another one."