In order to unlock the game's cheats, you have to collect
a certain amount of intel that's scattered around the game.
There's 30 in total, and with every milestone you complete, a new
cheat unlocks. Below is a list of the milestones, the number
represents how many intel files are needed to unlock a cheat.
2 - "COD Noire" - Black and white visuals.
4 - "Photo-Negative" - Inverted visuals/colors.
6 - "Super Contrast" - As it says.
8 - "Ragtime Warfare" - Plays like a silent movie.
10 - "Cluster Bombs" - One frag grenade = five.
15 - "A Bad Year" - We aren't sure. Something to do with exploding enemies.
20 - "Slow-Mo Ability" - Gameplay is in slow-mo.
30 - "Infinite Ammo" - Need we say more?
Beat the game once. Any difficulty works.
This is for multiplayer only. Get level 55 in multiplayer mode. Prestige Mode will be unlocked by letting you start from level one. Everytime you get to level 55, you will get new icons to demonstate your achievements. This can be done 10 times, for 10 icons.
Don't turn off the game during the credits. Let them finish. A secret mission on an airplane will begin right after they end.