Zyklon B gas, also known as hydrocyanic acid, is a poisoning gas that was used by the Nazis during the Holocaust. The gas comes in pellet form, and it vaporizes when it reacts to the air. The gas also had commercial uses as a disinfectant and insecticide.
After initial inhalation, the gas restricts oxygen from reaching red blood cells, which causes unconsciousness. Inhalation eventually leads to oxygen starvation, which is fatal.
During World War II, Nazis built gas chambers that looked like showers. They dropped the pellets, which sometimes had a blue tint and left behind a blue residue, in these chambers.