Women codebreakers knew some of the biggest secrets of WWII — including plans for the D-Day...

Before 150,000 soldiers landed on the beaches of Normandy for the military operation that would later be known as D-Day, few people outside the military knew what was in the works.

One of the rare people to know about the operation in advance was Jean Sims, 22, a codebreaker who had noticed the use of the phrase "Overlord" in the communique she decoded daily. Sims was one of thousands of "Code Girls," young women who volunteered to enlist with the U.S. Navy and worked to encode messages sent throughout the military and decode messages intercepted from enemy forces.