Tuesday is International Holocaust Remembrance Day, marking the passage of 70 years since the January 27, 1945, liberation of Auschwitz by Soviet soldiers. Auschwitz was a network of concentration camps built and operated in occupied Poland by Nazi Germany during the Second World War. Auschwitz I and nearby Auschwitz II-Birkenau were the extermination camps where an estimated 1.1 million people—mostly Jews from across Europe, but also political opponents, prisoners of war, homosexuals, and Roma—were killed in gas chambers or by systematic starvation, forced labor, disease, or medical experiments. About 200,000 camp inmates survived the ordeal. On Tuesday, a number of heads of states and aging survivors will attend a ceremony marking the 70th anniversary at the site in Poland, now maintained as a museum and memorial.