CIA Factbook
During the Cold War, monitoring the Soviet Union and its influence worldwide was the top priority for the CIA. In the 1950s, before reconnaissance satellites and other sophisticated collection systems were operational, wiretaps were one of the important technical means for collecting intelligence about Soviet military capabilities. The challenge was where and how to best conduct such wiretap operations.
Berlin was the center of a vast communications network from France to deep within Russia and Eastern Europe. At the time, almost all Soviet military telephone and telegraph traffic between Moscow, Warsaw, and Bucharest was routed through Berlin over land lines strung overhead from poles and buried underground. In a joint effort, the CIA and British Secret Intelligence Service (MI-6) assessed that tapping into underground communication lines in the Soviet sector of Berlin offered a good source for Soviet and East German intelligence. Tunneling from West Berlin to the underground cables in nearby East Berlin was judged to be feasible and hidden from visual surveillance.
Director of Central Intelligence Allen Dulles approved the covert tunneling and tapping operation in January 1954. Work began the following month using a US Air Force radar site and warehouse in West Berlin as cover for the construction.
Construction took a year. Tunnelers removed 3,100 tons of soil (enough to fill 20 average American living rooms) and used 125 tons of steel plate and 1,000 cubic yards of grout. The finished tunnel was 1,476 feet long. British technicians installed the taps, and collection began in May 1955.
Unknown at the time to the CIA and MI-6, the KGB—the Soviet Union’s premier intelligence agency—had been aware of the project from its start. George Blake, a KGB mole inside MI-6, had apprised the Soviets about the secret operation during its planning stages. But to protect Blake, the KGB allowed the operation to continue until April 1956 when they “accidentally discovered” the tunnel while supposedly repairing faulty underground cables—without putting Blake at risk. The Soviets planned the discovery in hopes of winning a propaganda victory by publicizing the operation. But their plan backfired when, instead of condemning the operation, most press coverage marveled at the audacity and technical ingenuity of the operation.
The taps produced enormous amounts of data for almost a year:
50,000 reels of tape
443,000 fully transcribed conversations
40,000 hours of telephone conversations
6,000,000 hours of teletype traffic
1,750 intelligence reports.
https://www.cia.gov/about-cia/cia-museum/experience-the-collection/text-version/stories/the-berlin-tunnel.html
