Operation Gold Operation Gold ( also known as Operation Stopwatch by the British ) was a joint operation conducted by the American CIA and the British Secret Intelligence Service in the 1950s to tap into landline communication of the Soviet Army headquarters in Berlin using a tunnel into the Soviet - occupied zone . This was a much more complex variation of the earlier Operation Silver project in Vienna . Soviet authorities were informed about Operation Gold from the very beginning by their mole George Blake and " discovered " the tunnel in 1956 . Overview . Although Operation Gold was planned by the SIS and the CIA , it was CIA money and manpower that carried it out . Details of the project are still classified , and whatever authoritative information can be found is scant . This is primarily because the then - Director of Central Intelligence , Allen Dulles ordered " as little as possible " be " reduced to writing " when the project was authorized . According to one account , Reinhard Gehlen , the head of the " Bundesnachrichtendienst " , first alerted Dulles to the location of a crucial telephone junction , less than two meters ( six feet ) underground , where three cables came together close to the border of the American sector of West Berlin . British and U . S . intelligence officials met in London to plan the tunnel . One of those who attended those early meetings was George Blake , a mole in the British intelligence apparatus . Blake apparently alerted the KGB immediately , for two of Gehlen ' s agents were caught trying to get a potential tapping wire across a Berlin canal . The KGB decided to let Operation Gold go on since in order to attack the tunnel , the Soviets would have to compromise Blake and they found it preferable to sacrifice some information rather than their valuable agent . The KGB did not inform anyone in Germany , including the East Germans or the Soviet users of the cable lines , about the taps . In December 1953 the operation was placed under the direction of William King Harvey , a former FBI official who transferred to the CIA . A warehouse with an unconventionally deep basement was specially constructed in the Rudow district of the American Sector of Berlin to serve as the staging area for the tunnel . The covert construction of a 450 - metre ( 1 , 476 - foot ) tunnel 6 metres ( ~ 20 feet ) under the world ’ s most heavily patrolled border to intersect a series of cable less than 47 cm ( 18 " ) below a busy street was an exceptional engineering challenge . Digging the initial vertical shaft for the tunnel began on 2 September 1954 and was completed on 25 February the following year . The tunnel ran underneath Treptow / Altglienicke in the Soviet Sector from an electronic box for the tapping of the wires in Neukölln / Rudow in the American Sector . There the British and the Americans listened and recorded messages flowing to and from Soviet military headquarters in Zossen , near Berlin ; conversations between Moscow and the Soviet embassy in East Berlin ; and conversations between East German and Soviet officials . It appears the West was unable to break the Soviet encryption at this time . Instead they took advantage of a faint electronic echo produced by the Soviet communications equipment to read the traffic in plain text . In Washington , DC , a team of CIA translators and analysts worked constantly on the vast amount of intercepts , ranging from high - level talk to barracks gossip . During the tunnel ' s brief lifespan of eleven months and eleven days , about half a million calls were recorded in 50 , 000 tapes . To evaluate this deluge the work of mining Operation Gold continued until September 1958 . The KGB , forced to keep the flow of information as normal as possible , would occasionally let authentic sounding communications get through ; the CIA may still have obtained some valuable information . When Blake received a transfer in 1955 , the Soviets were free to " discover " the tunnel . On 21 April 1956 , eleven months after the tunnel went into operation , Soviet and East German soldiers broke into the eastern end of the tunnel ; calling it a " breach of the norms of international law " and " a gangster act . " Newspapers around the world ran photographs of the underground partition of the tunnel directly under the inter - German frontier . The wall had a sign in German and Russian reading " Entry is Forbidden by the Commanding General . " Not only was Allen Dulles affected by the tunnel raid , but also his brother John Foster Dulles , the Secretary of State , and his sister Eleanor Lansing Dulles , the State Department ' s desk officer for Berlin . Only in 1961 , when Blake was arrested , tried and convicted , did Western officials realize that the tunnel had been compromised long before construction had begun . Although Allen Dulles has publicly celebrated the success of Operation Gold , CIA analysts have argued about the overall worth of the intelligence that they had gathered . By one assessment , the Soviets had allowed ordinary military communications to flow through the cables to project the illusion that the Soviets had no aggressive intent against West Berlin . Operation Gold forms the background to the novels " The Innocent " by Ian McEwan and " Voices Under Berlin : The Tale of a Monterey Mary " by T . H . E . Hill .