The concentration camps Auschwitz-I and Auschwitz II-Birkenau at Oświęcim, Poland were 1.1 million out of 1.3 million sent to the camps were killed (Jews, Poles, Gypsies, Homosexuals and Soviet POWs). From 1942 to 1944 during the holocaust several Jews were transported to the camps by trains from across German occupied Europe to be killed off in gas chambers using a pesticide Zyklon B. The most terrifying place I have ever visited.
