On July 16, 1945, the United States detonated the first atomic bomb near Alamagordo, N.M. Here is the color footage from the Trinity test site. (Gillian Brockell/The Washington Post)

A stone marker at the edge of town greets visitors with the official Los Alamos slogan: “Where Discoveries Are Made!”

This is a serene place these days. Children frolic by the pond in the center of the village. People sit at outdoor tables at the Starbucks across from the post office.

On Thursday, the Los Alamos National Laboratory, part of the Department of Energy, will commemorate the 70th anniversary of its greatest scientific accomplishment: the first successful test of an atomic bomb.

The anniversary of that explosion, which happened about 210 miles south of here at a site named Trinity, will be marked in a low-key fashion at the lab. There will be a roundtable discussion in an auditorium.

The participants will discuss, among other things, supercomputing. The lab doesn’t test nuclear weapons with actual explosions anymore; it’s done through computer simulations. The lab has a new supercomputer named Trinity, and a new slogan, “From Trinity to Trinity.”

A view of inside Los Alamos National Laboratory as researchers work on a nuclear testing project in 1974. (Atomic Energy Commission)

“The Trinity Test of 1945 was the first full-scale, real-world test of a nuclear weapon; with the new Trinity supercomputer our goal is to do this virtually, in 3D,” the lab declares on its Web site.

The Cold War has been over since the collapse of the Soviet Union in 1991, and the threat of nuclear armageddon has largely faded from public consciousness. Yet just in time for the 70th anniversary of Trinity, nuclear weapons have once again leapt onto the front page, with the controversial agreement between Iran and a U.S.-led coalition that is designed to push back a decade or more the development of an Iranian bomb.

The Iran deal is a reminder that these weapons remain very much a presence on the planet.

The casual visitor to Los Alamos can learn about the making of the bomb by stopping at the science museum run by the national lab, or going a block away to a small history museum. Coming soon: the Manhattan Project National Historical Park. Approved last year by Congress, this unit of the National Park Service will be based here and in the two other Manhattan Project sites, in Hanford, Wash., and Oak Ridge, Tenn.

Tracy Atkins, who manages the project for the National Park Service, said the park service isn’t worried that the Manhattan Project is too controversial: “As the nation’s storyteller, we deal with complex and controversial stories all the time.”

The practical problem is the security issue. People can’t just wander into the Los Alamos National Laboratory. The Park Service and the Energy Department are trying to figure out what lab properties can be opened up but, as lab spokesman Matt Nerzig points out, the historic sites are intermingled with operational facilities where nuclear weapons research is still being conducted.

‘Not the least bit confident’

Statues of two men, the founding fathers of Los Alamos, stand in the center of town outside the historic lodge where the scientists used to gather. One depicts J. Robert Oppenheimer, the brilliant physicist tapped to lead the laboratory. The other shows Brig. Gen. Leslie Groves, the Army’s commander for the Manhattan Project.

Scientists and workmen rig the world's first atomic bomb to hoist it in a 100-foot tower at the Trinity bomb test site in the desert near Alamagordo, N.M. in July 1945. (AP)

In 1942, they wanted to find a remote location for a secret laboratory. Oppenheimer loved the New Mexico desert and led Groves to Los Alamos, the site of a boys’ ranch. Los Alamos sits at an elevation of 7,300 feet on a finger mesa extending from mountains that frame an ancient caldera. The site was just 35 miles from Santa Fe but could be reached only by an unpaved, boulder-strewn road that repeatedly threatened to pitch cars and trucks into a chasm below.

Their secret laboratory was soon overrun with some of the most brilliant physicists on the planet, plus soldiers, military intelligence agents and a certain Klaus Fuchs, a German-born scientist secretly spying for the Russians. The isolated community also produced a lot of babies.

“All of a sudden it was basically a big maternity ward,” says Alan Carr, the laboratory’s historian.

The scientists and engineers drove themselves to exhaustion trying to make the bomb.

“They were not the least bit confident,” says Jennet Conant, author of “109 East Palace,” which describes the race to build an atomic bomb in the secret city known as “The Hill.” Conant’s grandfather, James Conant, was one of the leaders of the Manhattan Project.

“The weapon had never been built. It was theoretical science. . . . There was a huge leap from the science to the construction,” she said. “My grandfather had a full-on bleeding ulcer.”

‘Now I am become death’

By July 1945, the war in Europe had ended, but the Manhattan Project continued at a furious pace, with military leaders hoping to use the weapon against Japan and stave off the need for an invasion of the Japanese home islands.

In the days leading up to the July 16 test, the major elements of the bomb were assembled at the Trinity site and the bomb, nicknamed “The Gadget,” was lifted to the top of a tower. In the final, nervous hours, some scientists resurrected an old, discredited fear that a nuclear chain reaction might set the atmosphere on fire.

Roger Rasmussen is among the few people alive today who were at the Trinity site that morning. He’s 94 and still lives in Los Alamos. In 1945, he was an Army technician who, along with other soldiers, was just six miles from Ground Zero.

Lie facedown on the ground, they were told. Shut your eyes tight.

“I saw this light, tremendous light, which I presumed was the detonation. And I didn’t look up until I was told it was safe to look up. The radiation had passed us by that time. I looked up and I watched the fireball forming in the distance, a rather spectacular, multicolored, kind of a boiling ball.

“We were looking to the darkness of the west, and the lighting of the dawn was behind us. And so I watched the shock wave coming across the plain toward me. . . . It was a spectacular view, and it was boiling up at that time, and eventually turned into a mushroom cloud as the lighting improved after dawn.”

Watching, Oppenheimer quoted Hindu scripture from the Bhagavad-Gita: “Now I am become Death, the destroyer of worlds.” Physicist Kenneth Bainbridge spoke more coarsely: “Now we are all sons of bitches.”

Because now it was time to use it against human beings.

‘A tremendous debt’

On Aug. 6, the day the United States dropped the atomic bomb on Hiroshima, President Harry Truman revealed that, throughout the war years, scientists had been working to create “an atomic bomb.”

“It is a harnessing of the basic power of the universe,” Truman said. “The force from which the sun draws its power has been loosed against those who brought war to the Far East.”

Three days later a second bomb was dropped on Nagasaki. The two bombs took more than 200,000 lives. Japan soon surrendered.

Many of the scientists involved in the project were upset by the decision to use the bomb against civilians. Conant, the author, said the decision to bomb Nagasaki was particularly shocking to the Los Alamos community.

At the Bradbury Science Museum, the exhibit about the Little Boy and Fat Man bombs notes the horrible death tolls in Japan but also salutes the Los Alamos scientists:

“We must never forget the catastrophe that engulfed the world between 1939 and 1945. Los Alamos National Laboratory, which grew from the foundations of the Manhattan Project, salutes the men and women who came to this remote location to help the world win freedom from Nazi and Imperial Japanese tyranny. We owe them a tremendous debt.”

Down the street, the historical museum includes panoramic black-and-white photographs of the charred remnants of Hiroshima.

President Truman and his top advisers never regretted their decision to drop the atomic bombs on Japan. Defenders of the decision reasoned that it ultimately saved lives — not only American lives, but the Japanese lives that would have been lost in a U.S. ground invasion.

A spokesman for the lab, Kevin Roark, said he had an uncle, Melvin, who was on a ship in the Pacific when the bombs were dropped and the Japanese surrendered. “He was tremendously relieved, as was anyone who was in the Japanese invasion force,” Roark said.

Rasmussen, the Trinity test witness, will speak at Thursday’s roundtable, telling the story once again of that desert explosion 70 years ago. He speaks for many in his generation when he talks about the invention of the bomb: “Somebody was going to do it, and it was a race to see who did it first. And thank God it was the United States. And it’s been a deterrent ever since — that’s why it’s never been used.”

The atomic age isn’t over, though. The scientific knowledge that invented the bomb cannot be unlearned. This is a narrative without an ending. Two decades after the Trinity test, Robert Oppenheimer recalled that some people cried and some remained silent after that first explosion in the New Mexico desert:

“We knew the world would not be the same.”