Call of Duty's Zombies mode began as Nazi Zombies in Call of Duty: World at War in 2008, built around surviving endless rounds of the undead. Element 115, the fictional meteoric element behind the zombies, the teleporters and the mode's warped technology, became central to the storyline across later Treyarch maps. The distorted round-change sting is one of the mode's most recognizable audio cues, and clips of it spread through gaming montages, stream highlights and horror edits, where it still works as instant shorthand for escalating danger.