The cover story was radar. In the Rudow area of Neukölln, in the American sector, the US Army put up a warehouse with an ELINT station on the roof and a basement sunk to seven metres — far deeper than any warehouse needs, because it had to stage the dig and swallow the spoil. Digging began on 2 September 1954. William King Harvey, an ex-FBI man now with the CIA, ran the operation; Captain Williamson of the Army Corps of Engineers ran construction. The crews drove a tunnelling shield forward on hydraulic rams, lined the bore with sand and 1,700 cast-iron plates, and laid wooden track for rubber-wheeled carts. Three thousand tons of Berlin came out that way. At one point the diggers broke into an undocumented pre-war cesspool and flooded the works. The entire tunnel was wired with demolition charges that were never fired: Major General Charles Dasher would not approve an act that might start a third world war.

At the far end, 450 metres out and beneath Altglienicke in the Soviet sector, the target cables ran less than 47 centimetres below a busy street. Cutting the vertical shaft up to them was the most delicate work of the project, and it fell to Peter Lunn, head of SIS in Berlin and a former Olympic alpine skier. His people — Royal Engineers, SIS officers, and engineers from the British Post Office research station at Dollis Hill — improvised a device they called the Mole to work the shaft up through soil so crumbly that the smallest subsidence would have announced them to the street above. The splicing itself went to three Post Office cable jointers: Blake Rymer, reckoned the best jointer they had, working with Les Sparks and Arthur Loomes. Three cables, tapped without a flicker. The project had been costed at $50,000. It finished above $6.5 million, roughly the procurement cost of two Lockheed U-2s.

Western codebreakers could not read Soviet encryption in 1955, so the tunnel went after something simpler: people speaking carelessly on lines they believed were safe. The cables carried traffic to and from Soviet military headquarters at Zossen, between Moscow and the Soviet embassy in East Berlin, and between Soviet and East German officials. Roughly 90,000 communications were recorded. Sixty-seven thousand hours of Russian and German went to London, where a special section of 317 Russian émigrés and German linguists transcribed it, while multiplexed teleprinter signals went on magnetic tape to Frank Rowlett's Staff D. The KGB, remarkably, told nobody — not the East Germans, not even the Soviet officers using the cables. A CIA assessment found no known attempt to feed the operation disinformation. British intelligence suspected otherwise, but the Agency's own report concluded that the Soviet military kept using the lines for traffic of genuine intelligence value.

Blake transferred out of Berlin in 1955, and Moscow was finally free to act. On 21 April 1956, Soviet and East German soldiers broke into the eastern end. They called it a breach of the norms of international law and a gangster act, then invited the press underground to photograph the partition directly beneath the inter-German frontier, its sign lettered in German and Russian: entry forbidden by the commanding general. Western planners had assumed embarrassment would keep the Soviets quiet. Instead the world's newspapers ran the story as a triumph of engineering, and the CIA may well have gained more from the exposure than the Soviets did. Timing helped. Khrushchev was in Britain on a state visit, and the tunnel surfaced the day before his banquet with Queen Elizabeth II at Windsor Castle — which may explain why British participation went so lightly reported, even though most of the equipment in the photographs was British-built and clearly labelled.

Not until 1961, when Blake was arrested, tried and convicted, did Western officials grasp that the operation had been compromised before the first spade went in. Allen Dulles went on publicly celebrating what Operation Gold revealed about Soviet order of battle behind the Iron Curtain; a declassified NSA history is noticeably cooler about the take. The tunnel vanished under new construction. In 1996 the Berlin city government had roughly 83 metres dug out of the former American sector to clear ground for housing, and in 1997 a 12-metre section was recovered from the old Soviet side under the supervision of William Durie. That piece is displayed at the Allied Museum. Outer shell elements went to the CIA museum in 1999 and later to the International Spy Museum in Washington. Ian McEwan turned the affair into a novel, The Innocent; John Schlesinger filmed it.