Call of Duty Zombies began as a bonus mode in World at War in 2008 and grew into one of the franchise's most recognisable experiences, built around surviving waves of the undead in enclosed maps. Its harsh, growling audio became part of that identity, instantly signalling danger to players who had spent rounds holding off a horde. Clips from the mode later spread through gameplay videos, streams and soundboards, where they were reused far beyond the original matches. The sound remains popular because that tension is immediately recognisable, even to viewers who have never played the mode themselves.