With the renewed interest in nuclear weapons I have been struck by how few people there still are who have seen one explode. There are a few survivors of Hiroshima and Nagasaki and there are a small number who witnessed some of the above ground test explosions. But the last American above-ground test was in 1962 and the last above-ground test by any country was conducted by the Chinese in 1980. This means that the Indians, Pakistanis, Israelis—to say nothing of the Iranians and North Koreans—have never seen a nuclear explosion. In the main, this is a very good thing: the fallout from such a test is a real health hazard. But there is a downside. We have lost the experience of watching a nuclear explosion—perhaps the most powerful lesson about nuclear bombs there is.

In the spring of 1957, Kenneth Bainbridge, chairman of the physics department at Harvard, where I was finishing a two-year post-doctoral appointment, asked me if I would be interested in spending the summer at Los Alamos. I had no interest in working on nuclear weapons. But I had a great curiosity about Los Alamos, which was, at the time, a closed city surrounded by barbed wire. When I arrived I was assigned an office with Ken Johnson, who was also a Harvard postdoc. The weapons work seemed to be going on elsewhere; we were told nothing about it. I had an idea for a project in elementary particle physics but needed help to carry it out. Ken, who died a few years ago, was a very powerful mathematical physicist, so we worked on our problem.

My other activity was playing tennis on the Los Alamos cement courts. My partner was usually a senior physicist named Francis Low. Toward the end of August Francis said that he would not be available to play for the next week; he was going to Mercury, Nevada to watch some bomb tests, at the invitation of the head of the Theory Division, a Canadian named Carson Mark. I asked if there was any chance of my going too and on the morning of August 30, Francis, Carson, and I got into a light plane that flew from Los Alamos to Albuquerque and then by commercial airline to Las Vegas, which was about 65 miles from the test site.

My ignorance about how nuclear weapons actually worked was nearly total. I simply went where I was told to go, asking nothing. The plane was met by a government car that took us to a Las Vegas casino where we killed a couple of hours playing blackjack. Then we were driven to Mercury, an hour or so away, where we got a couple of hours of sleep. When Carson got us up it was still dark and quite chilly. He took us to a nearby meteorological station. Mercury had its own wind patterns, which were very important. If the fallout blew in the wrong direction it might irradiate Las Vegas. Then we went outside to a kind of concrete structure to await the test.

I recognized Al Peaslee, a Harvard graduate student, who had gone off to Los Alamos to work. Peaslee said that just after the explosion I should turn away for several seconds and put the smoked lenses I had been given over my eyes. He also said that I should expect the “shock wave.” I did not know what he meant. I learned from Peaslee that the bomb that was going to be tested was called “Smoky” and came from Edward Teller’s laboratory at Livermore. The Livermore devices were named after mountains while the Los Alamos bombs were named after scientists. That is all he told me.

In the distance I could see the tower, on top of which was the bomb. This tower was the tallest one that had ever been built for such a test, about two-thirds the height of the Empire State building. It had been deliberately placed in front of a bank of hills covered with Joshua Trees— so that the bomb’s effect on this kind of terrain could be studied. An alarm sounded and the countdown began. At zero there was an ungodly flash of light, which I could see reflected off the wall in front of me. I counted off a few seconds and turned around.

What I saw defies description. The photograph above gives some sense but not of the scale. At first there was no noise. Then came the shock wave that made a disagreeable click in my ears and finally the rolling thunder of the noise. The Joshua trees were aflame as if in some obscene pagan rite. The bomb had evaporated the tower. The fire ball rose and above it was a dark and very menacing radioactive column. It seemed to come towards us and I wondered if we should seek shelter. Above it was the mushroom cloud. We were all very silent when we returned to our bunkhouse for a little more sleep.

Sometime in mid-morning I heard the sound of helicopters. I recall saying to Carson without knowing the meaning, “They’re flying,” and his responding, “They’re flying and flying.” We had a bit of lunch and Carson took us on a visit around the site. The places where there had been explosions looked like the surface of the Moon, except that there were warning signs about the level of radioactivity. The first stop was at a five-hundred-foot tower where “Galileo,” a Los Alamos device, was being readied for the next morning’s test. There was an open lift that took us some feet below the platform on which Galileo had been placed. We had to climb a somewhat rickety ladder to get there. I remember looking down at the scrub desert somewhat anxiously until it occurred to me how ludicrous was this touch of acrophobia in comparison to approaching a bomb nearly as powerful as the one that destroyed Hiroshima.

Carson did not explain where we were going next: a concrete building quite far away that was set in a bunker. He opened the door and inside on shelves were the interiors—the “pits”—of a vast array of nuclear weapons. Carson casually picked one off the shelf and handed it to me. It was about the size and weight of a bowling ball—a bowling ball made out of plutonium and, in this case, with an outer layer of beryllium. It was slightly warm to the touch from the radioactivity. In the middle of the room there was a table. On it was a pit and a man was gluing high explosives to the sphere. A woman was next to him knitting. It was an uncanny scene about which I understood little. The next morning we watched Galileo explode. This time I knew what to expect but the experience was still overwhelming. Then we returned to Los Alamos.

Over the years Francis, who died in 2007, said very little about our experiences watching these two explosions at Mercury, so I do not know how they affected him. I know how it affected me. I was never quite the same. I cannot think of nuclear weapons as an abstraction. I listen to debates on nuclear proliferation and wonder if these people really understand what they are debating.

But I set out to learn, as the data became unclassified, about just what I had seen. Here is what I found out. Operation Plumbbob was a series of twenty nine tests nearly all above ground. They had begun on May 28, 1957 with “Boltzmann” and ended on October 5 that year with “Morgan.” The series, which was the most extensive ever done at Mercury, put 58.3 million curies of radio-iodine into the atmosphere. One-thousandth of a curie is what would be used in a liver scan. The radioactivity went all over the United States, with clusters in places like Maine. It is estimated that these tests caused some 38,000 thyroid cancers leading to about 2,000 deaths. The health burden of these tests put enough pressure—despite the protests of people like Edward Teller—to bring a halt to them. The same information can be gathered from an underground test witnessed only by mechanical devices.

It took some years before I understood the significance of the helicopters we heard that morning. In 1957, it was the height of the Cold War. The possibility of a ground war in Europe with the Soviet Union was taken very seriously. It was expected that a confrontation with the Soviets would go nuclear so troops were being trained for this eventuality. There were thousands at Mercury: fifteen hundred of them had been assembled about thirteen kilometers from ground zero of Smoky. By 6 a.m. they had been transported by helicopter to ground zero itself. They had very poor radiation monitors. This was to accustom them to nuclear war. Of these fifteen hundred soldiers some got leukemia and died.