British intelligence undertook an audacious operation to listen in on the private conversations of 10,000 German prisoners of war without their ever knowing they were being overheard. The prisoners' unguarded reminiscences and unintentional confessions have only just come to light, and prove how closely the German army were involved in the atrocities of the Holocaust. British intelligence requisitioned three stately homes for this epic task, and converted each into an elaborate trap. The 100,000 hours of conversation they captured provided crucial intelligence that changed the course of the war, and revealed some of its worst horrors, from rape to mass executions to one of the earliest bulletins from the concentration camps. But when the fighting ended, the recordings were destroyed and the transcripts locked away for half a century. Only now have they been declassified, researched and cross-referenced.