Treyarch's bonus zombie mode was meant as a special, throwaway time destroyer when it was injected into Call of Duty: World at War. "It's born out of the passion of the team," Treyarch boss Mark Lamia says at a recent preview event showcasing more multiplayer content and teasing the co-operative zombie killing mode.

Though its popularity has grown with each Call of Duty release tasked to Treyarch - the team behind this fall's Call of Duty: Black Ops 2 - the developer has never attempted to attach a fiction to the mode. As Lamia tells us, the fervent zombie mode fan community has taken on that job for itself.

"The story is kind of crowd-sourced. It's this phenomenon. It was amazing, when we put it out, people wanted to attach a story to it and you saw them start to do it." Fans began to link elements from the zombies mode, Lamia notes - including everything from textures used in the mode to weapons strewn about - in order to connect it to the franchise's established narrative.

With its most aggressive fans busy thrusting themselves into the role of fan fiction auteur, Treyarch continues to expand the zombie play space. So much has its concept for the mode grown, it's hard to imagine we won't one day see zombies as its own standalone game in the future. Until then, the latest iteration of the zombies mode is peppered with the same ambition we've seen scattered throughout the rest of Black Ops 2.