National Geographic Channel (NGC) journeys back to Nazi Europe to tell the story of Hitler's Hidden Holocaust -- the killing frenzy of Hitler's extermination brigades, known as the Einsatzgruppen or "action groups." Woven together with harrowing testimonials from survivors, witnesses and experts, this one-hour special presents in chilling detail -- with photographic evidence and rare video footage - how Nazi soldiers planned, documented and committed these horrific crimes. It was the same routine: Go into a town, round up Jewish families, take them to a ditch and shoot them, often in front of curious spectators..