Acid present in some plants due to hydrolysis.
1941. Adolf Hitler supposedly orders Heinrich Himmler to liquidate the Jewish population, known as the Final Solution. Initially, supposedly behind German lines on the Russian front, jews were simply shot where they stood. The mental strain on the Einsatzgruppen, or Elite Guard, was too much and the Reich searched for a faster way.
The original gas chamber was a building 93sq. meters. It was decorated with flower pots, and painted with a Star of David, and the guards told the jews it was a bath house. They packed naked jews into the room, 700 or 800 at a time, and piped in fumes from a diesel engine outside. It usually took about half-an-hour to exterminate all the people inside. Most were so packed together that they died standing.
The Jews were not being killed fast enough, so a new chemical was introduced. Prussic acid is extremely lethal in correct doses, and the Nazis perfected its application.
They employed the use of Zyklon B, which leaves hydrogen cyanide behind. Zyklon was produced in different concentrates, denoting a different price. The lowest was Zyklon F, and Zyklon D was used to exterminate rats, or other resilient vermin. Zyklon B was the lethal concentrate necessary to exterminate a full grown human in an open chamber.
Initially, Heinrich Himmler predicted the extermination of 20-30 million Slavs and Jews, but if I remember correctly they only got about 10 million. The bodies were examined inside and out, and stripped of all valuable possessions. Then dumped into mass graves, or burned. Looks like Saddam Hussein has been doing some homework...