One of the deadliest chemicals ever invented, Zyklon B is a derivative of Prussic acid. It was invented in 1922 by a small team of German chemists led by Nobel Prize winning chemist Fritz Haber, whose previous creations included mustard gas and other chemicals of warfare used in World War One.
In 1941, the gas was first deployed in three death camps: Auschwitz-Birkenau, and Majdanek, and Sachsenhausen. Its first large scale use was one September 3, when 600 Russian POWs, 250 Polish POWs and 10 criminals were killed at Auschwitz-Birkenau. Some of the victims survived more than 24 hours of exposure to the gas – when this was discovered, additional quantities of it were pumped into the killing chambers. By the time the war ended, an estimated 1.2 million people were killed with Zyklon B, most of whom (960,000) were Jews.