To unlock Extinction Mode, it seems you’ll need to do one of the following:

• Unlock Method #1: Finish the first Mission of the Singleplayer Campaign on any difficulty level. Then Extinction Mode will appear next time you go back to the Main Menu.

• Unlock Method #2: Or reach Level 5 in your Multiplayer Career. Then Extinction Mode will appear next time you go back to the Main Menu.

Once unlocked the on-screen menu will read: “Continue the Ghosts experience and save the world from the unknown threat in Extinction Mode.” And you can start playing the new mode.