Ted Petry's first job was in a secret laboratory at the University of Chicago, where scientists were trying to split the atom.

It was 1942, and nuclear physics was barely more than an idea discussed in university lecture halls, so the 18-year-old Petry was fuzzy on the details. He didn't ask for clarity. He was just a day laborer on the project, and he did what he was told. He liked the job mainly for its proximity to his parents' home on the South Side of Chicago and for the monthly salary.

After the experiment succeeded, the scientists packed up their equipment and moved to other facilities — Argonne, Los Alamos. Petry, too, sought new work, first on a freighter on Lake Michigan, then in a factory making airplane engines. When the war ended, he got married and had four kids. “I went on with my life,” he said.

Petry didn't think much about his old job until two decades later, when he was invited to an anniversary event at the White House. As President John F. Kennedy spoke about the “historic occasion” and the need to make sure it was applied “in the direction of the progress of mankind,” Petry's memories coalesced.

The atomic age was born in that underground lab, and he was there to see it.

Now 93, Petry is the last surviving witness of the world's first controlled nuclear reaction, 75 years ago Saturday.

On that otherwise ordinary December afternoon, cadmium control rods were removed from a pile of uranium spheres encased in graphite bricks. Neutrons fizzed off the uranium and bombarded other atoms, causing their nuclei to split. The fission, in turn, released more neutrons and sparked more reactions in a self-sustaining chain.

The energy generated wasn't sufficient to power a lightbulb. But in three years, the same technology, packed inside bombs, would level two cities. Today, experts measure humankind's proximity to global catastrophe based partly on the number of nuclear weapons in nations' arsenals.

Ted Petry stands beside a plaque marking the spot on the University of Chicago campus where the first controlled nuclear chain reaction was achieved in 1942. (Jean Lachat)

Petry was told none of this when he was recruited from Tilden Technical High School, along with his twin brother and several other boys. His employee records describe him as a laboratory assistant who made $94 a month, but he really was more of a “gofer,” he recalled recently, assigned to whatever tasks needed strong arms and quick hands.

Some days, he helped stack bricks of graphite in the 20-foot beehive-shaped “pile” that would form the reactor. Other times he was sent to a lab downtown to collect small containers of uranium for the project. He rode the streetcar with the radioactive material in his pocket, until a doctor noticed that his red-blood-cell count was getting low. The university scientists then provided him with a station wagon and a radiation-proof lead box.

The young men working on the project came and went. As World War II intensified, several, including Petry's twin brother, were drafted and sent overseas.

“Nothing was said about, 'Hey, we're working on a secret thing, these guys should be deferred,'" Petry said. “It was just a plain job that you went to and did what they asked you.”

Of course, the scientists running the experiment were keenly sensitive to what was at stake. Three years earlier, Albert Einstein and Leo Szilard, a Hungarian émigré who first conceived the idea of generating power via nuclear reactions, penned a letter to President Franklin Delano Roosevelt urging the United States government to invest in nuclear research. At that point, no human had ever controlled a nuclear chain reaction, let alone weaponized it. But Einstein and Szilard were certain that Germany was actively pursuing an atomic bomb.

By 1942, a team that included Szilard and Nobel Prize winners Enrico Fermi and Arthur Compton had assembled in Chicago to build the world's first nuclear reactor. When a labor strike stalled work in the Argonne Forest, 25 miles from the city, Compton suggested that the pile be moved to one of the squash courts beneath the University of Chicago's long-ago abandoned football field.

Compton, a professor there, didn't tell the university's president about the project even though a blunder could have been catastrophic in the densely populated area. Fermi was confident he could control the reaction; if anything went awry, he told his colleagues, “I will walk away — leisurely.”

The first controlled splitting of the atom was achieved at 3:35 p.m. Dec. 2, 1942, when Enrico Fermi and his team initiated the first self-sustaining nuclear chain reaction in a University of Chicago laboratory. (Department of Energy)

Working in 12-hour shifts, the team labored around the clock to construct the pile. The black dust of the graphite blocks formed a slick coat on the men's skin and the squash court's floor. The unheated workrooms were bitterly cold, but the mood was feverish.

In a 1982 article for the Bulletin of the Atomic Scientists, physicist Albert Wattenberg recalled grim conversations about what would happen if they succeeded — and the consequences if they didn't. He remembered fellow physicist Alvin Graves showing up several hours early one morning for his shift, explaining “he just couldn't sleep. He felt the Nazis were working, that they were pushing ahead to get there before us,” Wattenberg wrote. “We were in a real race and he felt he shouldn't be taking a day off.”

On Dec. 2, they were finally ready. Petry and 48 others crowded onto the squash court, where the pile now stood 20 feet high. A three-man “suicide squad” was ready to douse it with a cadmium solution that would stop the reaction if something went wrong. Fermi positioned himself at the base, calling out orders and making calculations with a slide rule.

With the cadmium control rods removed, the neutron count began to rise, until the reaction was self-sustaining. The pile had gone critical.

No one cheered or applauded, Petry recalled. But for the steady clack of the Geiger counters and the movement of detectors, there was no outward sign that anything had happened. After 28 minutes, when the radiation risk from the reaction became unsafe, Fermi ordered the control rods dropped back into the pile.

And then it was over, Petry said. History had been made.

The “birth certificate” of the Atomic Age. The galvanometer chart indicated the rise in neutron intensity associated with the first controlled chain reaction. (Department of Energy)

Eugene Paul Wigner, a theoretical physicist who helped build the pile and would later win a Nobel Prize for his work on the atomic nucleus, pulled a bottle of Chianti from a paper bag. Everybody got a sip. The mood was solemn, Wigner later wrote. Though they had no proof the experiment could be harnessed for military purposes, the scientists could imagine the formidable potential they'd unlocked.

“We could not escape an eerie feeling when we knew we had actually done it,” Wigner wrote. “We felt, as I presume, everyone feels who has done something that he knows will have very far-reaching consequences which he cannot foresee.”

Then all 49 witnesses signed their names on the bottle's wrapper — even the 18-year-old “gofer,” who had only a vague inkling of what had just been accomplished.

This year, for the 75th anniversary of the experiment, the university invited Petry to revisit his old stomping grounds. The squash courts where the pile was built were long ago demolished, the campus utterly transformed. A large bronze sculpture marks the historic site.

“It was the start of all — of the atomic world, I guess you’d say that,” he noted. “Everything proceeded from that experiment.”

Petry is diffident about his own role. He doesn't give talks about the project or spend time contemplating how splitting the atom transformed the world. “I was just a part of it, a small part of it,” he said.

In the National Archives of Chicago, Wigner's Chianti bottle can be found in an old cardboard box. Many of the names on its paper wrapper have faded with time. But right in front, in big block letters, the words “TED PETRY” are still visible.

Read more:

Thousands of scientists issue bleak 'second notice' to humanity

The Doomsday Clock just advanced, 'thanks to Trump’: It's now just 2½ minutes to 'midnight.'

Nuclear bombs transformed the planet and set off the Age of Humans, geologists say