Game developers are a cheeky bunch. They have been known to hide everything from secret levels to full-length movies inside their games. Sometimes, they take things even further and bury fully-playable games inside other games.

I’ve put together a list of some of the best hidden games within games, including retro and arcade titles that are nearly impossible to find legitimately.

A soldier shooting an enemy in Call of Duty.

15DOOM Eternal

DOOM Eternalis the seventh installment in one of the most iconic game series ever created. The first game,DOOM, together withWolfenstein, pioneered the first-person shooter (FPS) genre, and all the sequels that followed have provided non-stop action for gamers everywhere. Even if you’ve never played any of the games, you’ve probably heard about the originalDOOMrunning on everythingfrom pregnancy tests to alarm clocks.

InDOOM Eternal, you can play both the originalDOOMandDOOM 2. To do so, you’ll need to head to the Doom Slayers' office and interact with the CRT monitor there. ClassicDOOMis only playable after you’ve beaten theDOOM Eternalcampaign, but you can accessDOOM 2anytime by inputting the password “FLYNNTAGGART.”

A rebel holding a gun and looking down on an enemy in Homefront the Revolution

Now, here’s a bit of lore for you: in all theDOOMgames, the character you play as is never named. Fans of the game just refer to him as Doomguy. However, in the Doom novels, the protagonist is called Flynn Taggart, and the fact that his name unlocks the classic DOOM game feels like a tacit admission that the developers kept the same name.

14Call of Duty: Black OpsandBlack Ops Cold War

IfDOOMgave birth to the FPS genre, thenCall of Dutyraised it. The games simulate military combat across different eras, from World War II to a dystopian future, and are easily some of the most popular in the world.

InCall of Duty: Black OpsandCall of Duty: Black Ops Cold War, you can take a break from slinging bullets to play aninteractive fictiongame instead. You’ll need to find the secret terminal where the game is hosted first, but once you do, type in the word “Zork” to start the game.

An treasure hunter fighting a man with a mask in Pitfall The Lost Expedition.

If you’re wondering what a game likeZorkis doing in a high-action game likeCall of Duty, I can’t help you there, but far be it from me to look a gift horse in its mouth.

13Homefront: The Revolution

Homefrontis another FPS, but this time, set in an alternate timeline where North Korea invades America. It follows the activities of the protagonist, Ethan Brady, as he joins the resistance and attempts to overthrow the occupying army.

WhileHomefrontisn’t particularly well-regarded, it does have one standout feature—a hidden, fully playable port ofTimeSplitters 2. UnlikeHomefront,TimeSplitters 2is a beloved FPS classic. Its story follows a space marine traveling through different time periods to retrieve crystals before an alien race can use them to destroy humanity.

A group of gorillas hanging on to each other.

AccessingTimeSplitters 2inHomefrontis a bit tricky. First, you’ll need to find the in-game arcade and load upTimeSplitters 2, which initially grants access to the first two levels. To unlock the full version, you’ll need to enter a special code which is different based on what device you’re playing on.

12Pitfall: The Lost Expedition

Pitfall: The Lost Expeditionis a platform adventure game that follows the exploits of Harry, a rough-and-tumble treasure hunter, as he explores the lost city of El Dorado. It was released on the PS2, GameCube, and Xbox, and although it didn’t exactly set the world on fire, it remains a solid game nonetheless.

Locked away insideThe Lost Expeditionare two extra games—the originalPitfall!, andPitfall II: The Lost Caverns, both of which were released on theAtari. You can access both games after beatingThe Lost Expedition, and completing some achievements.

An Animal Crossing town sign.

11Donkey Kong 64

If you’re a fan of arcade games, theDonkey Kongfranchise needs no introduction. But for those unfamiliar,Donkey Kongis an iconic Nintendo game featuring a barrel-hurling gorilla that helped define the platformer genre. InDonkey Kong 64, you step into the gorilla’s shoes, embarking on a quest to rescue his kidnapped family.

InDonkey Kong 64, if you collect special items like the Nintendo coin, the Rareware coin, and enough fairy photos, you may unlock both the originalDonkey Konggame andJetpac, a shooter game developed for the ZX Spectrum (a home computer developed in the ’80s).

Nightwing, Robin, Batgirl and Red Hood from DC comic walking side by side on a street.

10Animal Crossing

You’ve probably heard aboutAnimal Crossing: New Horizons, but it is only the latest installment in a very popular franchise. The originalAnimal Crossingwas released in 2001 for the Game Cube and, much likeNew Horizons, it’s a cozy social simulation game where you play a human living in a village inhabited by animal characters. The game is open-ended, and you may do anything fromchoresto fishing and socializing with the villagers.

In the originalAnimal Crossing, you can obtain up to 19 Nintendo Entertainment System games, includingExcitebike,Balloon Fight,Clu Clu Land,Donkey Kong, andSoccerfrom Nook’s Lottery and other places around the island. Once you get the games, you can play them from withinAnimal Crossingjust like you would on the console.

A big purple tentacle rising high up and a smaller green tentacle cowering.

9Gotham Knights

Gotham Knightsis a 2022 action-adventure RPG developed by Warner Bros Games and set in a universe where Batman is dead. Now it’s up to the rest of the Bat family—Batgirl, Nightwing, Red Hood, and Robin, to protect the crime-ridden city of Gotham. The game is noArkham City, but it has a decent story and a well-developed world.

While exploring the world ofGotham Knights, you can make a detour to the Belfry where you will find an arcade cabinet forSpy Hunter—a car combat arcade game developed by Bally Midway in 1983. In the game, Bruce dominates the leaderboard with a score of 300,000, but you’re welcome to try your hand at beating his score.

Castlevania: Dracula X box art.

8Day of the Tentacle

Day of the Tentacleis a point-and-click adventure comedy game developed by LucasArts in 1993. It is the sequel to the critically acclaimedManiac Mansionand follows the adventures of three friends as they work together to prevent a mutated purple tentacle from taking over the world. you’re able to play the fullManiac Mansiongame on a computer you find insideDay of the Tentacle.

7Castlevania: The Dracula X Chronicles

TheCastlevaniaseries, along withMetroid, helped define theMetroidvaniagenre.The Dracula X Chroniclesis a PSP remake of an older game in the series—Castlevania: Rondo of Blood.In the game,you play as vampire hunter Richter Belmont, battling through Dracula’s Castle to defeat the dark lord himself.

If you find the right items in the game, you can unlock the originalRondo of Bloodon which TheDracula X Chroniclesis based, as well as a remastered version ofCastlevania: Symphony of the Night.

Metroid: Zero Mission screenshot from the Wii U store.

6Metroid: Zero Mission

Metroid Zero Missionis another Nintendo game, this time, developed in 2004, and following the adventures of bounty hunter Samus Aran as she attempts to prevent the Metroids—hostile parasitic creatures, from taking over the universe.

TheMetroidgames are famous for requiring you to backtrack to unlock areas, but if you manage to completeZero Missionon any difficulty, you will unlock the original version ofMetroidthat was released on the NES.