It was one of the most spectacular espionage operations of the Cold War: British and American intelligence services dug a tunnel from West Berlin that extended around 330 meters into East Germany. From there, they were able to eavesdrop on hundreds of thousands of telephone conversations between Soviet military personnel from May 1955 until the operation was exposed in April 1956.This book reconstructs the dramatic events and adds a missing piece of the puzzle that lay undiscovered for decades in the archives of the Verein Berliner Unterwelten: a comprehensive document compiled for GDR Interior Minister Karl Maron contains previously unknown photos and reveals details about how the spy tunnel worked and how the Soviets tracked it down.