If you've beaten "Call of Duty: World at War"'s main campaign, then you've unlocked the the Nazi zombie mode.
The special mode allows four-player co-op and has gamers battling throngs of Nazi undead that become increasingly difficult with each wave.
Activision executive producer Daniel Suarez shared with Stephen Totilo what inspired the team to make the mode in the first place.
"It was one of the pet projects of one of the designers," he said. "The seed of the idea came out of casual [tower] defense games online. It was something that they [developer Treyarch] had been working on for a while and went back to when they had time."
And although the project was something to be saved for later, it became sort of an urban legend within the company. "Nazi Zombies was sort of this rumor, and everybody would be like, 'Nazi zombies? What Nazi zombies?'" Suarez said. "The designers would sort of chuckle in the back corner and be like, 'I don't know what that is.' And then one day it all of a sudden appeared in a build and it got so much traction internally."
In fact, the mode became such a huge hit within the company, they decided to lavish some public attention on it. "All of the QA guys, the marketing guys, even the PR guys became big fans," he said. "So we decided to save it and announce it [the week of Halloween on GameTrailers]. It was a project of love that a lot of guys had a hand in."
When asked how long the mode lasts, he said, "I've been told that it's infinite." At the time of the announcement, the developers said they made it through 23 waves. Can you beat that?