Many of you will buy Call of Duty: Ghosts and hop directly into the game's multiplayer mode, without ever touching the game's single player. Of course, for those that want to experience every Multiplayer mode available, like Extinction, will notice that it's not available to play upon hopping into the Multiplayer lobby, or anywhere on the main menu.
Extinction Mode will only be accessible after you beat the mission Ghost Stories, which is the very first mission in the game.
It's a clever way to get players to at least try out the main campaign mode, who would have otherwise ignored it altogether.
After you clear the mission, Extinction Mode will be available from the Main Menu under Squads.
Happy alien hunting!