Zyklon B was a hydrogen cyanide-based poison. The Nazis used it in their concentration camps during World War II.
In 1940, the Royal Navy seized cylinders of Zyklon B from a German U-boat transporting it to a concentration camp. Hobbes later pumped it into the water supply of the British War Tunnels in order to kill the Nemonites infesting it. (AUDIO: The Nemonite Invasion).