Anyone that's played a Treyarch-developed Call of Duty game since 2008's World at War has likely tried 'zombie mode'. What most people don't know, however, is the history behind how that gameplay mode came about.

As Jason Blundell, Treyarch's Director of Campaign & Zombies, explains, a certain animation of soldiers burning caught the eye of a group of designers working on World at War. Without the fire, the animation resembled the slow plodding along that you'd expect from the undead.

The designers started working with the animation as a side project and before long, multiple departments within Treyarch got involved. Piece by piece, the mode came together and it soon became apparent that they'd have to tell Activision about it.

Needless to say, the publisher loved it. Instead of marketing it as part of the game, the team quietly added it as a minigame after completing the regular campaign.

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