As you probably already know, Infinity Ward chose to not include the traditional zombie mode in their latest development of Call Of Duty Ghosts. However, they made up for this with a fun-filled alien survival challenge called ‘Extinction' which like zombies, can be played with up to 3 other players. Now the big issue with ‘Extinction' is that it is not unlocked right away and most people don't know what they have to do to unlock it. There are actually two ways you can go about unlocking the mode and each of them are easy to accomplish:
Method #1: Play the classic online multiplayer modes
If you want to unlock Extinction the fastest, then this is the option you'll want to take. By participating in the classic multiplayer modes like domination, team deathmatch, kill confirmed, or any of the others, you'll rank up your online character which is necessary if you want to unlocked Extinction. When you reach level 5 or higher, Extinction will become unlocked and you'll be able to play it both online or offline. This took me about 2 and a half games of domination to reach level 5 so realistically it took about 20-25 minutes total, but this will differ on your skill level and the game mode you choose.
Method #2: Play the single-player campaign
If you're not interested in playing the online multiplayer modes or maybe you just don't have an Xbox Live subscription then you need to start playing the campaign. After you have taken the time to progress through the first 4 levels on regular difficulty or higher, Extinction will be unlocked. The first 4 levels are Ghost Stories, Brave New World, No Man’s Land, and Struck Down.