Just one city block down from Checkpoint Charlie on Niederkirchnerstraβe sits the Topography of Terror, a museum intentionally positioned at the site of the former headquarters for various notorious Nazi organizations such as the Gestapo and the Einsatzgruppen (a contingent of the Schutzstaffel, or SS; these were the specialized killing squads that ran riot behind the advancing German lines in the conquered territories of the East, rounding up and killing in mass Jews, Romani and Sinti peoples, as well as other perceived political or biological threats). Running along the front of the main museum building still stands the longest extant