Zombies is a type of game mode that was intoduced first in Treyarchs Call of Duty: World at War, or simply WaW.
The first zombies map was Nacht Der Untoten, the only map before the map packs were released. This map is a small 2 story type of house, which is set to be the Airfield from the campaign. The other maps from the original WaW map packs include: Vurruckt (Asylum), Shi No Numa (Zombie swamp, or undead swamp), and Der Riese (The rising I think it translates to)
In Call of Duty: Black Ops, the Default maps are Kino Der Toten (A German theatre) 'Five' (Pentagon), and Dead Ops Arcade (A Top-down arcade mode)
The map packs are
First Strike: Ascenion; Soviet Space Shuttle launching facility.
Escalation; Call of the Dead: Cold siberian outpost, George Romero follows you around.
Annhilation: Shangri-la: Jungles, the 115 meteor easter egg is done here.
Rezerection: All the maps from WaW including 'Moon' kill lots of zombies with low gravity. Hells yeah.
From Der Riese, into CoD: BO maps there is a Pack-A-Punch machine. For 5000 points this turns your stuffty gun into an awesome gun with a pewpewpew sound. It also changes the name to somehing a billion times more awesome. Usually something with 115 in the name. (Due to 115 being to storyline behind zombies, and the PaP machine being powered by it.)
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