Activistion and Infinity Ward have made some of the most popular "Call of Duty" games in the entire franchise. However, these games suffered one major handicap against the Treyarch-inspired titles which was that it did not come with the survival-type game mode "Nazi Zombies" (later just titled "Zombies"). This game mode pit up to four players against horde after horde of undead soldiers. Now, the developers behind "Call of Duty Ghosts" have a monster fighting survival game all their own - and here's how you unlock it.

Extinction mode is not unlocked by default, according to a game wiki guide on IGN.

"Players must play at least one mission of the campaign or play multiplayer (unsure if number of matches is a factor) before it will appear. Simply play through the first level of the game, which takes only 10-15 minutes. After doing so, return to the main menu to find it along with Campaign, Multiplayer and Squads.

Some players have also been finding that Extinction will unlock if they play a few rounds of Multiplayer. Each case seems to be slightly different, but either way does unlock Extinction mode in the end."

"Our world, broken, abandoned, But far from empty," says the opening voice over in the launch trailer for the new four-player co-op game mode, featured below. "This is not the world we remember, but this is out home and they are trespassing."

From the trailer it looks similar to Nazi Zombies in that it works as a stand-alone game plot detached from the reality of whatever the game's campaign mode will be. In the game mode, you and your three friends will be responsible for protecting a nuke while it prepares to launch. You'll use various methods of defense against different types of aliens.

Have you played "Call of Duty Ghosts'" Extinction Mode yet? If so, tell us some of your early experience stories. Perhaps you could share some helpful tips with our readers in the comments below.