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Operation Gold, the Berlin tunnel
A secret tunnel under East Berlin to tap phone lines
The US and Britain dug a tunnel from West Berlin to reach Soviet communications cables. It was discovered in April 1956, and the intelligence had been known to the Soviets through a mole.
The CIA and Britain's MI6 approved the project in early 1954 and built the tunnel in about a year. It ran roughly 1,476 feet from the American sector into East Berlin, under a cover story of a radar station. Crews removed around 3,100 tons of soil and lined the tunnel with steel plates.
Taps on the cables began recording on 11 May 1955. The Soviets claimed to discover the tunnel on 21 April 1956 while repairing cables, about eleven months later. The KGB had learned of the plan during planning from George Blake, an MI6 officer working for them, and apparently let it continue so as not to expose him.
The haul was large: the CIA cites about 1,750 intelligence reports, 443,000 transcribed conversations, and thousands of hours of telephone and teletype traffic. Analysts judged the material genuine and useful for assessing Soviet and East German forces. Blake was unmasked in 1961, and he later escaped from a British prison in 1966. The tunnel is often cited as one of the largest technical collection operations of the early Cold War, even though Soviet knowledge of it limited what the Americans could do with the intelligence.