The British-American spy tunnel extended approximately 330 metres from West Berlin into the GDR. From there, the intelligence services intercepted important telephone conversations between Soviet military personnel from May 1955 onwards. In total, over 400,000 telephone calls were intercepted. The Soviet intelligence service ended the operation in April 1956. The story is well known in film and literature, but only almost. In fact, an important piece of the puzzle piece has been missing until now, slumbering in the´archives of Berlin's underground world. It is the comprehensive documentation that was produced exclusively for the GDR Minister of the Interior, Karl Maron. It was generally considered lost. In fact, it contains previously unknown photos and details about the discovery of this tunnel and how it worked. A real find. It is introduced and explained by Dietmar Arnold, who knows Berlin's underworld like no other, and Helmut Müller-Enbergs, who studies Berlin's spies, sometimes for counter-espionage, sometimes as a researcher at the Free University of Berlin.