Constructed in 1955 from the American zone of occupation in Berlin to the site of a cable duct under the Schönefelder Chaussee carrying 28 Soviet telegraphic and 121 military communication channels to Karlshorst, the Berlin tunnel provided the Central Intelligence Agency and Secret Intelligence Service with access to Warsaw Pact planning for Central Europe. The information was recorded on magnetic tape, which was flown daily to London for processing at a dedicated center in Regent’s Park staffed by specially recruited Russian linguists. The total take was recorded over three months on 50,000 reels of tape amounting to 368,000 Soviet and 5,000 East German conversations. Code-named STOPWATCH/GOLD, the project came to an end in April 1956 when the Soviets pretended to discover the tunnel for the first time. In reality, the plan had been
betrayed at an earlier stage by George Blake.