P.O. Box 1663 was listed as a Santa Fe, New Mexico address in 1943.

And over the next few years, about 300 babies had it listed as the place of birth on their birth certificate.

Because the real location was a secret.

Everything sent to that P.O. box ended up here:

33 miles from Santa Fe at a site also known as P.O. Box 180, Project Y, and Los Alamos, New Mexico.

A secret city had been built there.

And it was home to a community of scientists.

"Scientists of many nations."

The scientists who created the first nuclear bomb.

They lived a couple hundred miles from the site where their invention would be tested.

"New Mexico desert."

Trinity.

How did laboratory director J. Robert Oppenheimer end up building a town and testing the first nuclear bomb here?

Albert Einstein sent this letter on August 2nd, 1939.

He sent it to President Franklin Delano Roosevelt.

Drawing from the work of physicists Enrico Fermi and Leo Szilard, Einstein warned of a nuclear chain reaction in a large mass of uranium,

"The splitting of the uranium atom...”

which could lead to extremely powerful bombs of a new type.

Bombs that either side might develop and use.

After a couple of years of study as well as the American entry into World War II,

in June 1942, the Army Chief of Staff established a temporary headquarters at 270 Broadway in New York City.

The Manhattan Project had begun.

And it was called a new "Manhattan District" for the Army Corps of Engineers.

This map shows contemporary boundaries for Army engineer districts. Administrative areas.

The Manhattan District encompassed all these smaller districts because of its larger scope: to build an atomic weapon.

Less prominent secret locations included a nuclear reactor under a University of Chicago football field, the Alabama Ordnance Works for producing heavy water, and many others.

1942 and 1943 saw the establishment of three major sites.

It began with Oak Ridge, Tennessee, sometimes called Y-12.

A large plant for the enrichment of uranium and production of some plutonium.

Nestled between mountains, it became...

"...a city where 75,000 people worked in absolute secrecy on history's most sensational secret.”

Two other major locations were established in 1943.

The Hanford Engineer Works in Washington state was responsible for much of the production of plutonium.

The top of this water tower there read "Silence means security."

But Hanford and Oak Ridge were nothing without the third site.

The army needed a place to create the bomb.

This is the Los Alamos Lament, a poem sometimes sung, about life in Los Alamos, written by technical Sergeant Ralph Gates.

It begins "I'm just a PO number.”

Specific numbers vary.

The third verse reads

"He put us on a mountain outside of Santa Fe where the only sign of wildlife are GI wolves at bay.”

Oppenheimer, based in Berkeley had believed that a central lab was key.

While they considered Oak Ridge and Chicago as lab locations, neither was remote enough.

An option near LA wasn't isolated.

One closer to Reno could be hit by heavy snows.

General Leslie Groves Jr. of the Army Corps of Engineers ran the project.

Oppenheimer and Groves agreed that New Mexico offered the security of isolation as well as familiarity, since Oppenheimer had spent time in the area.

The ideal site sat on the Pajarito Plateau.

It was isolated but also protected by its altitude and surrounding geography.

Jemez Springs, chosen first, proved to be too difficult.

The land was too difficult to acquire and the terrain was too rugged.

But nearby Los Alamos was atop a table land between mesas which made it easy to control entry and control any accidents.

Much of it was on already federally-owned land as well.

The only existing structure was a small school that had opened in 1935.

The owners sold.

The Secretary of War wrote the Secretary of Agriculture about the military necessity of acquiring the remaining federally-owned lands.

The request was granted for 54,000 acres of a demolition range.

Los Alamos was activated on April 1st, 1943.

P.O. Box 1663 transformed from an outdoorsy ranch school with buildings like this into a community doing the most advanced research in the world.

Roads were quickly developed, but the town was kept isolated.

Population grew from 1500 people to 5700 by 1945.

So rapid that hutments were a common form of accommodation.

Here you can see the wash drying by Quonset huts.

Apartment buildings were also available.

These accommodations mingled next to facilities for graphite fabrication and the cyclotron and Van de Graaff machines.

In early years, Los Alamos housed the world's finest researchers.

Here, Dorothy McKibbin in charge of receiving new personnel, sits next to Oppenheimer.

He's chatting with physicist Victor Weisskopf.

Here's Enrico Fermi on a hike.

And this is Edward Teller's ID badge.

He was later called the father of the hydrogen bomb.

The Medical Corps colonel wrote Leslie Groves that this intellectual group created challenges for a military operation.

"The large percentage of intellectuals... will require and seek more medical care than the average person.”

Other challenges?

"One-fifth of the married women became pregnant in Los Alamos, making maternity wards a necessity."

The past and atomic future intersected.

Ice was cut from nearby ponds and stored in ice houses because electric fridges were too hard to get.

At the time, the Bendix washer was revolutionizing laundry.

By 1943, a classified ad in the Santa Fe New Mexican was looking for one to be shipped to P.O. Box 1663 for wartime work.

But cultural phenomena as varied as they were, like this Los Alamos band, they had one real purpose: Building a bomb.

And they needed a place to test the bomb that they built.

This is the base camp at Trinity site.

A rapidly established headquarters created for testing the first atomic bomb.

The desert training center north of Rice, California, was runner up.

But it wasn't isolated enough or close enough to Los Alamos.

Located in the Jornada del Muerto Valley.

The winning site was selected with a more extreme version of the Los Alamos criteria.

Flat terrain to minimize blast effects.

Isolated, yet close enough to Los Alamos.

Good weather and nearby to highways like US-85 and 380.

More than 200 residents settled at the camp.

First, there was a 100 ton explosives test in May 1945.

Then they prepared the Gadget nuclear device.

And on July 16th, 1945, they conducted the test.

"First try out of this new cosmic force was held on the New Mexico desert."

The Los Alamos Lament was written after that test but before the August bombing of Japan.

"I'm just a P.O. number...”

"I have no real address...”

"Although we were selected...”

"I wonder for the best...”

"We're not like other people..."

"No one knows what we do..."

"So P.O. Box 1663..."

"Here's to you."