Video Game Secrets and Easter Eggs

Explanation: The Konami Code (↑↑↓↓←→←→BA) was created by Kazuhisa Hashimoto for Gradius in 1986. It became one of the most famous codes in video game history.

Explanation: Adventure (1979) on Atari 2600 contains the first documented easter egg: by finding an invisible pixel, the player accesses a room displaying creator Warren Robinett's name.

Explanation: Akuma (Gouki in Japan) appears in Super Street Fighter II Turbo if the player reaches the boss without losing a round and without using a continue. He then defeats M. Bison and becomes the final boss.

Explanation: Using a jetpack to reach the top of the bridge, players discover a sign reading "There are no Easter Eggs up here. Go away." - a sarcastic nod from Rockstar to secret hunters.

Explanation: Level E1M9 "Military Base" is a secret level accessible from E1M3. However, E1M1 "Hangar" contains many secret walls revealing hidden weapons and ammo.

Explanation: At the top of a mountain in Skellige, you find the body of a man dressed in white with a hood - a direct reference to Assassins from the Assassin's Creed series, dead from a failed fall.

Explanation: The "Level Select" in Sonic 1 is activated by entering ↑↓←→ then holding A+Start at the title screen. This code allows access to all levels and the Sound Test.

Explanation: Graffiti left by former test subjects in Portal reveal that "The cake is a lie", which became one of the most famous memes in video gaming.

Explanation: In Day of the Tentacle (1993), you can play the entire Maniac Mansion (1987) by using the computer in Ed's room. A massive easter egg for its time!

Explanation: Naming a sheep "jeb_" (referring to Jens Bergensten, Minecraft lead developer) makes its wool permanently change color through the entire rainbow spectrum.

Explanation: In MGS: The Twin Snakes on GameCube, Mario and Yoshi figurines are hidden in Otacon's office. When aiming at them with the sniper, Otacon comments on them.

Explanation: The "Secret Cow Level" in Diablo II opens by combining Wirt's Leg and a Tome of Town Portal in the Horadric Cube. Hell Bovines and the Cow King await players there.

Explanation: Typing "how do you turn this on" in Age of Empires II spawns a blue Shelby Cobra equipped with a machine gun, capable of destroying any unit in a few shots.

Explanation: Stuffed monkeys are hidden on several Halo 3 maps. The most famous one is on the "Sandtrap" map where you need to use a vehicle to reach it.

Explanation: If you let Earthbound run for decades on the title screen (or modify the system date), a special message appears. In reality, after a few minutes of inactivity, the game starts a demo.