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Blue Sphere Opening Screen.

Blue Sphere is a hidden mini-game included on the Sonic and Knuckles cartridge, based on the Special Stages of Sonic 3 and Knuckles. It is playable when you place (almost) any Genesis cartridge other than Sonic the Hedgehog 2 or Sonic 3 and Knuckles into S&K lock-on slot and press the A, B, and C buttons at the same time. If you do this with any cartridge other than Sonic the Hedgehog or Sonic Classics 3-In-1, you'll only get to play a single stage.

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 Rerelease locked into the top slot of S3&K. It also appears as its own game option in Sonic Mega Collection and Sonic Mega Collection Plus (called Blue Sphere, despite not having an official title).

Each stage consists of four procedurally-generated blocks, in any combination, giving the game over 100,000,000 different levels.

The goal is to navigate a pseudo-3D environment and collect all of the blue spheres in the level, either by running through them and turning them into Red Spheres, or by encircling groups of Blue Spheres in loops of Red Spheres. For the entire level, you're stuck running on the grid between the tiles and can only turn in 90-degree angles on the intersecting lines. You have no control over your running speed, you can't stop running, and you'll automatically speed up at regular intervals, decreasing your chances of survival the longer you take.

The most common obstacle is the Red Sphere, and running into one means instant failure. (In case you're wondering, the Blue Spheres you transform don't become hazardous until you've passed through them.) Other obstacles include Star Bumper Spheres and Yellow Spring Spheres. Running into a Star Sphere makes you run backwards until you run into another one or push up to run forward again. Touching a Yellow Sphere bounces you up and lands you five grid units ahead (or backward, if you're running in reverse at the time).

Rings can also be found in Blue Sphere, but here they're individually worthless, providing no protection from Red Spheres. Collecting all the rings in a stage and then winning the stage gives you a "perfect" ranking, which skips you forward by 10 levels instead of the usual 1. However, most rings in each stage are hidden, and the only way to reveal them is to enclose groups of Blue Spheres within a loop of Red Spheres, at which point the grouped spheres turn into rings. In order to get every ring, you must transform every sphere that can become a ring. Messing up and hitting even one wrong sphere loses you the "perfect" ranking. And even if you manage to hit all the right spheres, if you hit the last Blue Sphere before collecting all the rings, the stage immediately ends and you progress a measly one stage instead.

Why yes, Blue Sphere is Sega Hard.

Not to be confused with Star Ocean: Blue Sphere.

Tropes used in Blue Sphere include:
  • Easter Egg: The game's actual existence outside of Sonic 3 and 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 3-1 cartridge was used.
  • No Plot, No Problem: Just press the buttons and go right in!
  • Palette 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.
    • There's a nifty newish hack that lets you play as the unusual combination of Knuckles and Tails, but again it makes little difference gameplay wise.
  • One-Hit-Point Wonder: Sort of. You can't die, but hitting the only thing that "harms" you (Red Spheres) makes you fail instantly.
  • 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 Without An S: Some sources list this game as Blue Spheres.
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