One of the most enduring mysteries in Hollywood is Greta Garbo’s retirement from acting in 1941. The notoriously reclusive star was one of the few actresses to make the transition to “talkies” successfully, but during the mid-30s she began taking less roles and mysteriously disappearing for months on end. During those times, Garbo was working with producer Alexander Korba and MI6 on a number of top secret intelligence missions. Her first task was to collect information on Swiss industrialist Axel Wenner-Gren, and she continued to work for the agency throughout the decade, reporting back on potential Nazi sympathizers and agents. Some say her work in Denmark was responsible for the survival of legendary physicist Niels Bohr.