- One summer day, an Alaska-based air monitoring crew received an alarming reading from their equipment.

They quickly telegraphed their superiors, notifying them that they had just registered positive radioactive evidence of a recent explosion.

Now to the everyday American in 1949, this might not have seemed like concerning news.

Atomic tests were so common that some were tourist attractions.

But this nuclear signature was different.

It came from the Soviet Union, and it came years earlier than expected.

This successful test only increased pressure on American intelligence operatives to solve a very dangerous problem.

Soviet infiltration of their nuclear program.

Faced with an unbreakable Soviet code, they turned to their best hope, a group of brilliant mathematicians.

I'm Joel Cook and this is "Rogue History."

The US Army's Signal Intelligence Service, predecessor to today's National Security Agency, launched the confidential Venona Project in February of 1943 to break encrypted Soviet diplomatic communications.

And yes, the Americans and Soviets were World War II allies in 1943, but it was really more of an enemy of my enemy sort of situation.

We'll get to that later.

The SIS needed talented mathematicians with the ability to commit full time to the grind of codebreaking.

Daily shifts lasted at least 12 hours, and employees sometimes worked seven days a week.

So 65 years before Beyonce did it, the Signal Intelligence Service called the single ladies, but it wasn't for the same kind of empowering reasons.

Sexist stereotypes at that time implied that single women wouldn't be occupied with being wives or mothers, and the SIS felt they would dedicate themselves completely to the project.

But recruiting was a little awkward.

How do you go about interviewing people for a job so secretive that not even the president knew about it?

The university students with high marks might be called into what they thought was a club meeting only to find themselves face to face with a uniformed military officer asking them to serve their country.

In small towns, newspaper ads and posters announced the arrival of recruiters looking for women who could "keep their lips zipped" about the nature of their work.

None of the Venona women knew they were signing up to be codebreakers for a classified project.

They knew only that their country needed them, and that with so many people sacrificing for the war effort, they were willing to do their part.

The first member of Venona was Gene Grabeel, a 23-year-old from the small town of Rose Hill, Virginia.

After attending college, she settled into a job as a home economics teacher, but housewife duties weren't really her thing, so she decided to apply for a vaguely described position with the government.

Another recruit, Angeline Nanni, joined Venona in 1945 after taking a complicated test at the top secret SIS facility known as Arlington Hall.

According to Nanni, the test consisted of sets of numbers arranged in 10 rows with five digits in each row.

The instructor told the recruits to subtract the bottom rows from the top rows while non-carrying.

And while I still don't really know what that means, Nanni definitely did.

She finished first in her test group, confirming to the SIS that she could handle the rigor of Venona project.

The first breakthrough for the Venona codebreakers came when the Finns passed a tip to the Japanese about Soviet indicators, special numbers that give clues on code systems.

Because the Americans were already secretly reading Japanese encrypted messages, they were able to steal the indicator information and pass it on to Grabeel and the others.

Stealing this information was incredibly risky, though.

If the Soviets found out, it could result in an international crisis, like a lose World War II level international crisis.

Fortunately, though, that didn't happen.

Allowing the Venona team to learn that there were five different Soviet communication systems, but that was about all they could do.

The complexity of Soviet code came from their use of the one-time pad system, which ironically was invented by an American.

In the Soviet system, the sender of the code would translate each word into four-digit numbers using a code book, then translate those numbers into five digits by moving the first digit of the second number to the end of the first number, and repeating that process until the end of the message.

Once this process was complete, the sender would then use the one-time pad, a booklet of randomly-generated five-digit numbers.

Using the same mathematical process Angeline Nanni was tested for, the sender would begin adding the numbers of the encrypted message to the numbers on the page of the pad using non-carrying addition.

The receiver would reverse the process.

After completion, the page was destroyed.

Because only the sender and receiver had a copy of the one-time pad, the code could not be broken, as long as they didn't make the mistake of reusing pages.

But the Soviets, in a rare miscalculation, did exactly that.

In 1944, a trio of codebreakers, Lucille Campbell, Genevieve Feinstein, and Cecil Phillips, found seven message groups with duplicate number usage.

Because of their breakthrough, it was possible to rebuild the Soviet code books from scratch, a type of decryption called book-breaking.

One of the most talented book-breakers in the SIS was Meredith Gardner, a linguist who worked with the Signal Intelligence Service as a Japanese and German codebreaker during the war.

Gardner was also fluent in Russian.

He built on the work of the original team by decoding words like into and the, and with the help of a Soviet code and cipher found in 1945, was able to decrypt more complex messages.

The next year, he discovered something chilling.

The Soviets had assets inside the Manhattan Project, stealing atomic secrets.

In the late 1940s, the United States was the only country in the world with nuclear weapons.

The Soviets hadn't quite figured them out yet and were estimated to be several years away from doing so.

So when they successfully exploded an atomic bomb in Kazakhstan in 1949, the need to identify the Soviet spies reached a fever pitch.

One of the highest priorities on the team's radar was a spy known first as Antenna and then as Liberal in Soviet decrypts.

He first appeared in 1947 in Meredith Gardner's "Special Report #1" with only his wife's name, Ethel, as a potential identifier.

But what the investigators could confirm for certain was that this Liberal was the leader of the atomic spy ring.

Venona decrypts pointed to him as the primary recipient of technological and scientific information stolen from the Manhattan Project.

They just needed to figure out who he was.

By September 1949, Venona successfully identified the first of the atomic spies, Klaus Fuchs, a Manhattan Project scientist referred to as Rest and Charles by the Soviets.

In his interrogation, he revealed the spy work of two other Manhattan Project employees.

When the FBI interrogated one of them, David Greenglass, he gave them a considerable amount of information that led to the arrest of his brother-in-law and sister, Julius and Ethel Rosenberg.

Based on the testimonies of implicated spies and decryptions from the Venona team, the FBI determined that Julius Rosenberg was indeed the man the Soviets called Liberal.

In the summer of 1950, he and Ethel were arrested and charged under the Espionage Conspiracy Statute.

However, the public was less convinced.

Because the Venona project was classified, the public was unaware of the evidence they had against Rosenberg.

They assumed that the charges were influenced more by the red scare, a period of intense paranoia about communist infiltrators.

Ethel Rosenberg's role in the spying was also lost in the secrecy of the Venona Project.

A decryption from Meredith Gardner implied that Ethel was aware of Julius's work but didn't participate very often because of her health.

To avoid alerting the Soviets to the codebreakers, this evidence wasn't admitted to the court.

But FBI interviews from David and Ruth Greenglass, likely given under coercion, were admitted and implicated Ethel as an active participant.

The Rosenbergs were convicted in 1951 and executed on June 19th, 1953, despite international outcry.

It makes you wonder what the outcome would've been if the Venona information was declassified at the time of the trial.

The Venona Project continued to work on Soviet code decryption for nearly 30 years after the Rosenberg trials.

Declassified in 1995, the records show that 349 Americans were proven to have relationships with Soviet intelligence agencies.

Venona also identified spies in nearly every agency in the federal government, with one even serving as an aid to President Franklin D. Roosevelt.

After the declassification of Venona, the credit for the project was given almost completely to Meredith Gardner, the male linguist and book-breaker.

While Gardner was certainly vital to the project, there were dozens of women codebreakers doing countless hours of work to set him up for success.

It's only been in recent years that the women codebreakers of Venona were acknowledged for their important contributions.

When we think about the events of the past, we often overlook significant contributors because they weren't working in the conventional roles we so often celebrate.

But for every battle won, for every spy ring stopped, for every conflict avoided, there were people in the shadows doing essential work to create those successes.

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