In 1955, Operation STOPWATCH/GOLD ran a tunnel, 800 metres long, under the Russian sector of Cold War Berlin to tune into Red Army intelligence transmissions. This was an audacious trick: apart from the technical wizardry, any noise or vibration could have given the game away.
But there was a mole in the British secret services, and the KGB knew about the tunnel even before it was built; and if the Red Army trusted the KGB it too was mistaken. Why the KGB kept this secret to itself is one of the many questions that Stafford explores. He is the first to reveal this whole, puzzling story, drawing on eye-witness interviews and a full range of sources.