In the early 1950s, American signals intelligence detected something anomalous: a persistent microwave signal being directed at the U.S. Embassy in Moscow, transmitted from locations outside the compound. The signal operated at frequencies between 2.5 and 4.0 GHz—standard microwave frequencies—and appeared to be deliberately focused on the building, particularly on windows and areas where diplomatic operations and sensitive conversations were known to occur. The power levels were relatively low, approximately 0.001 to 0.018 mW/cm², but they were sufficient to activate certain types of passive electronic devices if correctly designed.

American intelligence agencies faced a dilemma. The signal could serve one of three purposes, or possibly a combination of them: it might be an attempt to activate a concealed listening device placed inside the Embassy, similar in concept to Leon Theremin's Great Seal microphone; it might be a form of electronic harassment or attack intended to damage equipment or disrupt operations; or it might serve other purposes entirely, such as illuminating sensors or transmitters placed by Soviet agents. The U.S. government discovered the signal in the early 1950s but did not reveal its existence to Embassy personnel, treating the signal as a classified security matter.

The CIA launched an investigation into the signal, codenamed Project PANDORA in 1965, attempting to characterize the signal's properties, understand its purpose, and determine what threat it posed to the Embassy and its occupants. The Defense Intelligence Agency and the Office of the Secretary of Defense also initiated studies, bringing in specialists in microwave physics, bioeffects, and electronic countermeasures.

Throughout the 1960s, American ambassadors served in Moscow unaware that they were being subjected to microwave radiation. Charles Bohlen, Llewellyn Thompson, Foy Kohler, and others conducted their diplomatic missions in offices and residences that were being targeted by focused microwave beams. The radiation levels, while low, exceeded American occupational safety standards by significant factors, though they remained within Soviet safety standards.

Ambassador Walter Stoessel, who served in Moscow from 1969 to 1974, was eventually informed about the signal by American intelligence officials. Stoessel's response was marked by concern but also resignation; he chose to remain at his post despite the health risks. During his tenure, Stoessel developed a rare blood disease, a fact that later became the subject of intense speculation about whether the microwave exposure was causally related to his illness. The State Department issued Stoessel a written warning about the potential health risks from the microwave radiation in 1976.

Studies conducted by American medical researchers found elevated rates of cancer, leukemia, and other diseases among Embassy personnel compared to control groups of American diplomats who had served in other Soviet Bloc countries without microwave exposure. However, these studies were unable to establish definitive causation, in part because the exposed population was small and the diseases in question have multiple etiologies. The inconclusiveness of the research studies led to decades of controversy about what dangers, if any, the signal posed.

By the mid-1970s, tensions over the Moscow Signal had escalated significantly. The Soviet government did not acknowledge the existence of the signal or respond to American diplomatic protests about it. American intelligence officials became increasingly convinced that the signal served some offensive purpose, whether activating hidden listening devices or harming the health of diplomatic personnel. In 1976, after more than two decades of bombardment, the American government issued a strong diplomatic protest to the Soviet Union. The signal ceased operating shortly thereafter.

In response to the signal, the U.S. government installed extensive copper mesh shielding in the Embassy to create a Faraday cage effect, blocking external electromagnetic radiation from reaching sensitive areas. Upgraded electronic countermeasures were deployed, and the diplomatic sections of the Embassy were reinforced against microwave penetration. These expensive countermeasures represented acknowledgment, by action if not by explicit policy statement, that the threat posed by the Moscow Signal had been taken seriously.

The Moscow Signal remained one of the most sensitive and disputed incidents of the Cold War. Official records remain partially classified. The true purpose of the signal—whether it was designed to activate listening devices, to damage equipment, to disrupt communications, or to conduct health effects research on Embassy personnel—has never been definitively established by declassified documents. What is certain is that for twenty-three years, American diplomats in Moscow conducted their most sensitive negotiations while Soviet microwave beams scanned the building they worked in, a tangible reminder of the pervasive espionage that characterized U.S.-Soviet relations during the Cold War.