Numbers: Thundergun: Use a Thundergun in the campaign, simply on the first part destroy all Nova 6 barrels with a grenade or gun, there should be six of them. After the player shoots them all, Nova 6 will start coming out of them and the screen will shake. Then go to the back of the room and find the open tape dock. Hold the use button to pick up the cassette. The screen will shake again after this step. Continue to escape. When the player reaches the first armory behind the fridge, find the open tape dock on the left and hold the use button again, it will close. If the player did this right, then after a few seconds the screen should shake. Wait a few more seconds and hold the use button once again. The player should now have obtained the Thundergun.
Nuketown: Hidden Song: If the heads of the mannequins are decapitated within 15 seconds, "Sympathy for the Devil" by The Rolling Stones is played. Requires multiple players to achieve. This may be hard, as the mannequins are randomly placed on the map every round, some might even spawn outside the map itself.
The names on the mailboxes are the names of the protagonists in the campaign, Mason, Hudson and Woods. They will also have other names on them, which may be of some of the developers.
The "Welcome to Nuketown" sign will change population according to how many players are currently in the game, and the clock tower will cont the minutes left in the game
In the building on the top right of the map, there's a cargo transport board showing the names of all the maps in the game, including the First Strike and Escalation packs.
There are "Wanted" posters all over the map of a boy named Raffaele Angelini at the age of 14. It says that he's wanted for "pwning noobs". It also says that his occupations are "Pwning noobs and being awesome" and that he's a very good Call of Duty player. Further down it says that he's very loved by the developers at Treyarch and Activision Blizzard. According to the description, he may be the son of one of the developers.
In the police station, posters can be found that says "Men with long hippie hair will be attended to last". Under it says that long hippie hair is: "Hair touching the eyebrows" or "Hair falling across the forehead" or "Hair covering the ears" or "Hair reaching below an ordinary shirt collar". There are also picture illustrating these.
In the lower left house there's a rack of magazines named "Exposed", which on the front page says "Russians launch first primate into space, and it returns a... WHAT THE HELL IS THIS?" with a picture of a Space Monkey from Ascension. The same magazines is found on the third mission, Executive Order.
In some buildings there are pictures of an early version of Hotel and the map Stockpile.
Note that the Zombies Mode easter eggs fromWorld at War are also found in the Black Ops remakes.
Nacht Der Untoten
Undone (hidden song): Plays after shooting several barrels outside.
Hidden song plays when you shoot the radio near the Mystery Box in the Help Room.
Lullaby for a Dead Man (Hidden song) Activated when one of the toilets are flushed 3 times.
Dentist's chair: In one of the first rooms you start out in, there is a dentist's chair in the center. Hold the action button while looking at the wheel on the base of it and you will hear a dentists tool running and a man screaming.
Shi no Numa
Radio: A radio which explains information about 115.
The One - Dialing the telephone 3 times in the hut via Comm Room Section.
Kino der Toten
115 (hidden song): Plays a song when 3 pieces of meteor are activated.
Film Reels/Radios: 3 film reels can be found and played on the projector, and 3 radios (one of which has not been found) can be activated to play recorded messages.
Telephones: Plays Eminem's "Won't Back Down" when all three red phones are activated. (Note if you have GCF enabled you will not be able to listen to this song due to lots of strong language. Instead it will be replaced with constantly repeating music from the zombies main menu)
Dead Ops Arcade
The hidden zombie game mode itself, Dead Ops Arcade, is unlocked by entering "DOA"(Or 3ARC UNLOCK but this unlocks all the campaign and zombie maps) in the CIA Data System.
Casimir Mechanism: Long easter egg requiring four players and Wonder Weapons; gives everyone a Death Machine for 90 seconds.
Abracadavre (hidden song). Plays after three teddies holding sickles are activated.
Character Dolls: Matryoshka dolls can be found that look like the original Nazi Zombies characters, which also talk, the quotes vary from player to doll, meaning there is a total of 16 quotes from the dolls.
There are several telephones across the map that will sometimes ring. When answered, character quotes from "Five" can be heard.
Call of the Dead
Original Characters Trapped: A variable length (Short in Solo, Long in Co-Op) easter egg which sends the usual Zombies characters to "Paradise". Unlocks an 2 achievements for doing it in Co-op, as well as an individual one for Solo only.
Not Ready to Die (hidden song): Hidden song by Avenged Sevenfold. Activated by Holding the Action Button on 3 Meteor Fragments in the first 3 areas.
Radios: Play messages recorded by Richtofen.
Eclipse: Another very long easter egg in which you free Gary And Brock.
Pareidolia (hidden song): Plays a song when 3 pieces of meteor are activated.
Coming Home (hidden song): Plays a song when 3 Teddy Bears are activated.
Re-Damned: Hidden song which is activated by pressing the "use" button on a computer next to the Bio-Dome's door.
8-bit Pareidolia: Hidden song which is activated by obtaining a disc and using it on a computer in the bottom floor of the labs.
8-bit Coming Home: Hidden song which is activated by obtaining a disc and using it on a computer in Tunnel 6.
Nightmare (hidden song): Activated by having somebody die when an Excavator is down, and then finishing the round, resulting in them spawning on the other side. (Completing One Giant Leap)
Richtofen's Grand Scheme: A long easter egg which consists of two parts, which must be completed in the following order:
1) Cryogenic Slumber Party: An easter egg where it requires the player to complete a number of steps to release Sam from the Pyramid, when the pyramid opens, all players will be granted a Death Machine for 90 seconds.
2) Big Bang Theory: An easter egg which requires the players to complete a number of steps to launch three rockets to Earth. After they hit Earth, all players are granted with permanent eight perks. (Unlocks an Achievement on 360 and PC)
Here's a relatively obscure Easter egg that I might just be imagining. In the mission "Crash Site", you'll pass a bunch of soldiers on your way to the river boat. Some are snapping photographs, some are guarding prisoners and some are walking patrol. If you take a closer look at the latter group, you'll notice that two of these soldiers – Private Hammond and Private Clarkson – share their names with hosts of Top Gear, a popular British TV series about cars.
Also, these two holes in Nuketown's fence are no mere decoration. The RC-XD can drive through them and explore areas beyond the normal limitations of the map. Forget your kill count – there's a ramp out there!
To see the Thor's Hammer Easter Egg in the campaign you need to start the mission "Celerium" and get past the portion of the game when you fly in. Once you've flown in, eliminate all resistance in the area, walk to the cliff area and drop down to the lower area. There is a dark cave nearby and inside you can see Thor's Mjolnir Hammer.
A computer accessible from the main menu room allows you to browse the contents of a CIA computer and enter various cheat codes. This allows you to access Dreamland and play Zork, which unlocks an achievement. Type in Zork to play it, or you can play Dead Ops Arcade by typing DOA. See the video above for instructions.
I described this shortcut in a separate post, but this deserves a mention all by itself. So just to make sure all cheaters are satisfied… yes, you can type "3ARC INTEL" into the game's hidden computer terminal to instantly unlock and uncensor everything from the confidential Intel section of the main menu.