Video Game: Blue Sphere

Blue Sphere opening screen

The Sonic & Knuckles cartridge is designed to Lock-On with a Sonic 3 or a Sonic 2 cartridge. It will not work with non-Sonic cartridges.
— the misleading disclaimer in the Sonic & Knuckles instruction manual

Blue Sphere is a hidden minigame included on the Sonic & Knuckles cartridge, based on the Special Stages of Sonic 3 & Knuckles. It is playable when you place (almost) any Genesis cartridge other than Sonic the Hedgehog 2 or 3 into S&K's lock-on slot and press the A, B, and C buttons at the same time. The vast majority of Genesis cartridges you can do this with only give you a single stage. note  If, however, you do this with the first Sonic game or Sonic Classics 3 in 1, you'll have access to a whopping 134,217,728 stages. note 

Upon completion of the stage, you'll be given a twelve-digit password for use when you play the game with the original Sonic the Hedgehog or the aforementioned compilation re-release locked into the top slot of S3&K. It also appears as its own game option in Sonic Mega Collection, Sonic Mega Collection Plus (called Blue Sphere, despite not having an official title) and the Sonic and Knuckles Collection for the PC.

Not to be confused with Star Ocean: Blue Sphere.

This game provides examples of:

  • Difficulty By Acceleration: You move faster the longer you take to complete the stage.
  • Dungeon Bypass: Getting every ring in a stage in the full version of the game to get a "Perfect" skips you 10 stages ahead.
  • Easter Egg: The game's actual existence outside of Sonic 3 & Knuckles. Technical difficulties with the palette prevented Sonic Team from doing Knuckles in Sonic 1, so they put this game in. The game's existence and the button combination required to play it weren't mentioned in the manual, which in fact said not to put in any other game.
  • Endless Game: After a while, certain levels repeat themselves and eventually just carries on from Level 1 again. Only if you put in the specific cartridge needed to play the full game, though.
  • Nintendo Hard: Some of the combinations are devilish, especially when running for perfects.
  • Password Save: There are two kinds of passwords. One for the stage as presented in the main game, and one for the version of the stage where a non Sonic 1 or Sonic Classics 3 in 1 cartridge was used. The former allows you to advance the game, whereas you're stuck with the password's level with latter.
  • No Plot? No Problem!: Just press the buttons and go right in.
  • One-Hit-Point Wonder: If you touch a red sphere, you instantly lose.
  • Screw Destiny: The manual says that putting any game other than the second or third Sonic game won't work. You do it anyway. And then you press all the buttons for no reason.
  • Schmuck Bait: Scroll back up.
  • Spell My Name with an "S": Some sources list this game as Blue Spheres.
  • Sprite Swap: You can play as both Sonic and Knuckles (some mods let you play as Tails), but there's no difference. All you can do is run and jump, and the speed you move at and your jump height is out of your control.