Secret cities. Secret work. During World War II, about 130,000 people were involved in the Manhattan Project, a highly classified collective effort that produced the first nuclear weapons and changed science, and war, forever.

But those working on the project didn’t necessarily know what they were working on. Many of the oral histories at “Voices of the Manhattan Project,” an extensive collection of interviews, come from people who understood that their work was of strategic importance but didn’t realize they were helping create an atomic bomb.

Anne McCusick, who purified uranium at Oak Ridge, didn’t realize she was contributing to a nuclear weapon.

“We didn’t know that they were going to drop a bomb. We didn’t know that if they dropped the bomb it wouldn’t be on some isolated atoll,” she said in 2011.

“We figured it out on our own,” said Dieter Gruen, a chemist who had been told to refer to uranium as “tube alloy.” “No one told us. No one.”

Others knew more than they could ever reveal.

Physicist Lawrence Litz was followed on a vacation by government agents intent on making sure he didn’t spill secrets about the bombs’ cores. And Army engineer Richard Yalman recalls his wife asking questions he couldn’t answer after she discovered his job required regular urine tests.

The project, which began in 1942, was an enormous undertaking. It required figuring out how to produce an entirely new substance, plutonium, and how to mine and enrich enough uranium to fuel a bomb. New instruments were invented and fabricated; scientists recruited and moved; calculations made, checked and rechecked. Physicists, engineers and others moved en masse to places such as Los Alamos, N.M, Oak Ridge, Tenn., and other sites around the United States, Canada and the United Kingdom.

Participants’ stories range from humorous to sobering; most engage with both personal histories and the role of science in creating history’s most powerful weapons. Read and listen to 580 oral histories — preserved in a partnership between the Atomic Heritage Foundation and the National Museum of Nuclear Science and History — at