A section of an ingenious tunnel built by U.S. and British spies to intercept Russian phone conversations in Cold War Berlin has been found after 56 years in a forest 150 kilometers from the German capital.

The 450-meter-long tunnel, built in 1955, led from Rudow in West Berlin to Alt-Glienicke in Soviet-occupied East Berlin. By tapping into the enemy’s underground cables, Allied intelligence agents recorded 440,000 phone calls, gaining a clearer picture of Red Army maneuvers in eastern Germany at a time when nuclear war seemed an imminent threat.

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