The Zombies, also known as Nazi Zombies because Nazis are their most common form, are the titular main enemies of the secret game mode in Call of Duty: World at War, Call of Duty: Black Ops, Call of Duty: Black Ops II and Call of Duty: Black Ops III. They were reanimated by an extraterrestrial chemical known as "Element 115", which came to Earth in meteors. Though zombies were originally featured in the Treyarch video games, they have also appeared in Call of Duty: Online, Call of Duty: Advance Warfare, Call of Duty: Infinite Warfare, and Call of Duty: WWII.