Unlocking this bonus mode won't be easy or quick like entering in a code, however. You must beat the single-player campaign and wait for the credits to end before a single zombie steps in front of your gun.
The mode pits you and up to three friends in a boarded up mansion under siege by wave after wave of the undead. Each subsequent wave will be naturally harder to defeat, which is why you'll need a little additional help.
As waves are defeated new areas of the mansion will open up. In this new areas you and your co-op buds will be able to buy new more powerful weapons and ammo, and build barricades to slow down the infestation.
It is our understand that there is no end to the zombie plague. Waves keep attacking until every member of your squad drops dead. The goal is to reach the highest level and score "humanly" possible while the zombies grow increasingly difficult to take down.
If you needed some additional incentive to sink a few hours into beating Call of Duty: World at War, this should help tip the scale.
Call of Duty: World at War is now available in stores everywhere for Xbox 360 and Playstation 3 in regular and collector's edition flavors (view them all). Let the zombie hunting officially begin.