SOVIET SPIES (DURING THE COLD WAR)
https://en.m.wikipedia.org/wiki/John_Anthony_Walker#cite_note-4
I think the biggest that we know about to date is John Anthony Walker Jr. (July 28, 1937 – August 28, 2014) was a United States Navy chief warrant officer and communications specialist convicted of spying for the Soviet Union from 1968 to 1985 and sentenced to life in prison.
Along with SCPO Jerry Whitworth.
In late 1985, Walker made a plea bargain with federal prosecutors, which required him to provide full details of his espionage activities and testify against his co-conspirator, former senior chief petty officer Jerry Whitworth. In exchange, prosecutors agreed to a lesser sentence for Walker's son, former Seaman Michael Walker, who was also involved in the spy ring.[2] During his time as a Soviet spy, Walker helped the Soviets decipher more than one million encrypted naval messages,[3] organizing a spy operation that The New York Times reported in 1987 "is sometimes described as the most damaging Soviet spy ring in history."[4]
After Walker's arrest, Caspar Weinberger, President Ronald Reagan's Secretary of Defense, concluded that the Soviet Union made significant gains in naval warfare attributable to Walker's spying. Weinberger stated that the information Walker gave Moscow allowed the Soviets "access to weapons and sensor data and naval tactics, terrorist threats, and surface, submarine, and airborne training, readiness and tactics."[5] John Lehman, Secretary of the Navy during the Reagan Administration, stated in an interview that Walker's activities enabled the Soviets to know where U.S. submarines were at all times. Lehman said the Walker espionage would have resulted in huge loss of American lives in the event of war.
Jerry Alfred Whitworth (born August 10, 1939) was sentenced to 365 years for his part in the Walker family spy ring,[2] which, at the time of Whitworth's arrest, U.S. authorities described as "the most damaging espionage ring uncovered in the United States in three decades".
Walker went so far as to involve his children in his spying by having them enlist in the Navy once he was retired.
