The Great Seal Bug (also known as the Thing) was a Soviet mission in which a listening device was implanted within a copy of the Great Seal of the United States which was presented as a plaque to the the United States Ambassador to the Soviet Union W. Averell Harriman.
The plaque was presented On August 4, 1945 by a delegation of the Young Pioneer Organization of the Soviet Union, a youth organization for Soviet youth aged 10 to 15. It was presented in friendship as a peaceful gesture towards the Soviet’s World War II ally. Unbeknownst to the ambassador, the plaque contained a listening device that was hidden in holes drilled under the beak of the eagle in the plaque. The device was extremely thin and powered remotely by an electromagnet energy source making it very difficult to detect. It was designed by Leon Theremin, a famed Russian inventor. The microphone hidden inside was passive and was only activated when the Soviets decided to turn it on. They usually did so from a van parked outside of the ambassador’s house in which they ultra-high frequency beams at the house. Activated, the microphone would transmit any conversations coming from within range of the bug. When they turned the beams off, the bug was virtually undetectable.
Ambassador Kennan later revealed in his memoirs that the Spasso House had been redecorated under Soviet supervision without and American presence. Surveillance sweeps were done routinely but revealed nothing. He also said that technicians conducting the surveillance asked him to pretend to be working out of the house one evening instead of his office. He therefore summoned his secretary to the Spasso House and began dictating a fictitious letter. At one point, one of the technicians encouraged him to continue with the dictation and began sweeping the room quietly. Eventually his sweep led to the Great Seal plaque which e took apart and found the bug.
The Soviets were undeterred and continued planting bugs throughout the rest of the Cold War.