The ultra-secret ‘Berlin Tunnel’ was dug in the mid-1950s from the American sector in southwest Berlin and ran nearly a quarter-mile into the Soviet sector, allowing the CIA and the British Secret Intelligence Service (SIS) to tap into critical KGB and Soviet military underground telecommunication lines. George Blake, a trusted officer working in a highly sensitive job with SIS, was privy to every aspect of the plan. Over the course of eleven months from May 1955 to April 1956, when the Soviets discovered the tunnel, ‘Operation Gold’ provided seemingly invaluable intelligence about Soviet capabilities and intentions. In a work of remarkable investigative reporting, Steve Vogel now reveals that the information picked up by the CIA and SIS was more valuable than even they believed. But why would the Soviets, knowing full well that the tunnel existed, have let slip many of their most valuable secrets?