In late 1942, President Franklin D. Roosevelt and the United States government developed a top-secret plan. The goal: to develop atomic bombs -- and beat Nazi Germany in doing so. The name: The Manhattan Project.

The unprecedented scientific project would span 4 years and 14 sites nationwide, with famous names like Los Alamos and Oak Ridge, and ultimately lead to the creation of the atomic bomb. It was to include 130,000 workers, 6,000 scientists -- and one young physicist named Robert Carter.

Carter was just a graduate student at Purdue University when he received a letter in December 1943 inviting him to join his professors in Los Alamos.

They couldn't tell him what the project was. They couldn't say what he would be doing. They just asked him to show up.

“I went out not knowing where I was going or what I was going to be working on," Carter told FoxNews.com. "But I trusted these gentlemen,” he said. Now 91 years old, the Bethesda, Md, resident says he's one of the last living people to work on the Manhattan Project. But he couldn't say that at the time.

“We were told not to even tell people we were going to Los Alamos,” he said, to prevent anyone from finding out about the project. If family or friends wanted to write to them, “all of our mail was sent to Santa Fe.”

Carter realizes now the stunning significance of the work. But there was no way he could know anything in 1943, when he was asked to leave his studies at Purdue for “classified war work.”

He knew the physics department had taken on a confidential assignment. The senior professors “all went out West. They couldn’t tell me where they were going, except for out West.”

Within 24 hours of his arrival, Carter found he would be working under J. Robert Oppenheimer on the Manhattan Project, helping with the research and experimentation that led to the first atomic bombs. For his service, Carter would earn a civil servants wage of around $2,500 a year.

Carter told FoxNews.com he felt privileged when he found out what he was doing. “I felt honored but surprised -- I thought I could contribute.”

The physicist worked as a technical assistant experimenting with nuclear reactors, using uranium. And despite the magnitude of the accomplishments, the day-to-day work was often quite ordinary: Carter spent his time on the project working in a lab, trying to discern how big the nuclear explosions would be.

“Some of the measurements I helped do were directly related to the size and configurations to the nuclear cores of the plutonium bombs,” he said.

Hundreds of people worked with him in the secretive Los Alamos facility, he said. Carter praised Oppenheimer’s vision. “He gave the impression of being very capable of doing whatever he chose to do.”

Carter witnessed the explosion at Trinity, a site in rural New Mexico where an experimental bomb was set off. It produced great results. 

“The magnitude of the whole thing was overwhelming as it detonated, sitting there in the desert in the early morning,” he told FoxNews.com. Two other historical bombs were produced at Los Alamos: Named Fat Man and Little Boy, each weighed around 4 tons. The pair were set off over Nagasaki and Hiroshima in Japan, ending World War II and killing more than 100,000 people.

While at Los Alamos, Carter observed the construction of the inner core of Fat Man.

“I’m so pleased it was successful, but so sorry it was used against people,” he said. Though the consequences were devastating, the work was certainly remarkable, and a stunning scientific achievement.

 “To achieve this,” he said. “It’s pretty special. It’s hard to describe -- it was a once in a lifetime opportunity.” Ultimately, the work on the project changed his life, Carter said.

After World War II ended, in the winter of 1946, he continued his graduate studies at the University of Illinois. Carter received his master’s degree in physics and returned to Los Alamos to continue the research.

“I intended to get a Ph. D. and teach at a small college someplace, but I decided research was a lot more fun.”

He met his wife in Los Alamos and they had 11 children. Today Carter lives on his own outside of Washington, D.C.; he has 30 grandchildren, and 11 great-grandchildren.

After leaving Los Alamos, Carter and his family moved to the Washington, D.C., area, where he worked for the Department of Defense, researching the biological effects of nuclear weapons at war. He also spent time working for the Nuclear Regulatory Commission.

He’s returned to Los Alamos on many occasions, most recently last summer with his family, when he visited the Los Alamos National Laboratory.

At Los Alamos, the research on security and technology continues.