The Zombies mode is Treyarch's ace in the hole. What began as a fun extra at the end of CoD: World at War has quickly blossomed into a legitimate third gameplay pillar that stands shoulder to shoulder alongside the multiplayer and campaign modes.
It was also the first indication that Treyarch were so much more than a 'me too' studio', fit only for toiling away on World War II shooters. Free of the confines of real-world conflict, Zombies has allowed Treyarch to take Call of Duty into uncharted territory - taking in sights such as Nazi Zombies, Sarah Michelle Gellar v George Romero and 'Space Monkeys'.
In one memorable scene, we even get to step foot on the moon! As much as there is to like about Infinity Ward's games, we can't say they've ever taken us into space. Well, maybe once, briefly. But then they blew us up.
Zombies is now an essential part of the Call of Duty tapestry, and try as it might, Infinity Ward's Spec Ops mode just can't compare. We can't wait to find out where this year's installments will take us. How can you ever possibly hope to top Space Monkeys? For now at least, the answer lies locked in a climate-controlled vault, deep within Treyarch HQ.