First passive listening device

The first passive remote listening device (a microphone that works with no active electronic components) was "The Thing", which comprised a metal cylinder with a thin membrane on one end and a long metal rod extending from the side. The Thing was discovered by US counterintelligence officers in 1951 during a sweep of the US embassy in Moscow. It was hidden inside an enormous wooden carving of the Great Seal of the United States, which had been gifted to the ambassador by a delegation from the Soviet Young Pioneers youth group in August 1945.

The Thing was developed in the mid-1940s by scientist and electronic music pioneer Leon Theremin. He had been working for the Soviet secret service (the NKVD) on-and-off since the early 1930s, but had fallen foul of the NKVD after a possible attempted defection to the United States in 1938. He invented The Thing (along with the "Buran" infrared microphone system) while interned in a research-station prison camp.

The mechanism of its operation was simple: when a radio transmitter was directed at the antenna using a frequency of 800 mhz, the antenna would become energised, turning the metal cylinder into a resonant-cavity microphone. The sounds from the room would then be returned as a weak radio signal. The advantage of this design was that it only transmitted a detectable signal when it was being actively "illuminated" by a remote radio transmission – at all other times it was entirely passive.

The Thing was not discovered until a radio operator accidentally picked up the signal from the device while returning their receiver. It was handed over to British counterintelligence officer and engineer Peter Wright (a man often described as the real-world equivalent of James Bond's "Q") who reverse-engineered the device. The discovery was kept secret until 1960, when in the aftermath of the Gary Powers U-2 incident the US ambassador to the UN brought out the seal (complete with listening device) to demonstrate that the spying was mutual.