CNN  — 

How many people does it take to build an atomic bomb? About 125,000, if you count the inhabitants of three entire US cities that were built from scratch in 1943 as part of the Manhattan Project.

The cities, which also served as testing grounds for new architectural principles, were Los Alamos in New Mexico, Oak Ridge in Tennessee and Hanford/Richland in Washington state.

It was all done in secret. They weren’t on any maps, and almost none of the residents knew they were working on a new type of bomb, only some kind of war effort. For a casual observer, these cities were normal places with the occasional oddity: Every baby born in Los Alamos, for example, had a P.O. box in Santa Fe listed as their birth place.

But on August 6, 1945, when the bomb nicknamed “Little Boy” was dropped on the Japanese city of Hiroshima, the true nature of these cities was revealed to the world – and to the people who lived in them.

No man’s land

These cities were designed to house the massive facilities needed to refine radioactive material and construct the weapons themselves. To accommodate them, the US government started to quietly acquire land in the fall of 1942.

“The secrecy was impressive,” said Martin Moeller, curator of the Washington D.C. exhibition “Secret cities: The Architecture and planning of the Manhattan Project,” in a phone interview. “The existence of these cities was never acknowledged by the government during the war.”

The choice of sites was far from random. They were scarcely populated areas, which made it easier to secure the land and keep evictions to a minimum. And they were close – but not too close – to an established transport hub.

An aerial view of the road to Los Alamos.The sites selected all took advantage of natural barriers to enhance security and secrecy.

“All of them were about 25 to 35 miles from existing population centers – far enough that, in the 1940s, you could get out of people’s hair, but not so far that you couldn’t get people to train stations relatively close by,” said Moeller.

The official reason given for the displacement was the construction of a demolition range, most likely to encourage people to leave for fear that their homes would be damaged. Once the cities and facilities were built, a number of other false rumors were circulated, including one that attributed developments to the production of ammunition.

Moeller guesses that just a few hundred people in the country knew about the bomb before it was dropped. Nothing was explained to the tens of thousands who lived and worked in the cities that produced them, working “like moles in the dark,” as Life magazine put it in 1945.

“Obviously people knew something was afoot, but they didn’t really know what it was,” Moeller said.

“And even if they were working on uranium, no one had ever seen or heard of a nuclear weapon before. But it’s hard to put ourselves in the mindset of the 1940s. At the time, most people wouldn’t question anything in the name of the war effort.”

Architectural legacy

Although built quickly, the three cities proved hospitable. Few of the accommodations resembled military barracks or temporary housing.

“The leaders of the Manhattan Project felt that in order for things to proceed smoothly, workers and particularly scientists and engineers needed to feel at home,” said Moeller.

One of the most common houses in Oak Ridge was the B-1 model, commonly known as the Flat Top.

“So they embarked on an effort to build single family houses, winding roads and planned communities with green spaces. Some of the people who lived there, even during the war, talk about what a great place it was to grow up.”

But not everything was so progressive: Racial segregation was built into the planning from the start.

“(It happened) not only in Oak Ridge, which was in the south, but also in Los Alamos and Hanford, very different parts of the country, culturally,” said Moeller. “In all three cases, the general assumption was that segregation was a given. So, particularly at Hanford, not only African-American workers but also Latino workers were segregated.”