[[quoteright:320:https://static.tvtropes.org/pmwiki/pub/images/no_way_7.png]]
[[caption-width-right:320:NO WAY?[[note]]Get Blue Spheres![[/note]]]]

->'''IMPORTANT'''\\
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 ''[[VideoGame/Sonic3AndKnuckles Sonic & Knuckles]]'' instruction manual.

''Blue Sphere'', also sometimes officially named ''[[InconsistentSpelling Blue Spheres]]'', is a hidden minigame included on the ''Sonic & Knuckles'' cartridge, based on the Special Stages of ''VideoGame/Sonic3AndKnuckles''. It is playable when you place (almost) any Genesis cartridge other than ''VideoGame/SonicTheHedgehog2'' 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]]Some of the larger Genesis games, specifically ones larger than 2MB, are incompatible and just result in you going to ''Sonic & Knuckles'' instead.[[/note]] If, however, you do this with [[VideoGame/SonicTheHedgehog1 the first Sonic game]] or ''[[CompilationRerelease Sonic Classics 3 in 1]]'', you'll have access to a whopping 134,217,728 stages. [[note]][[http://info.sonicretro.org/Blue_Sphere Reportedly]], recycled levels start showing up after stage 128,016,000. Not that you'd even be able to get that far without passwords, anyway.[[/note]]

The object is to collect the Blue Spheres scattered throughout the wrap-around stage. Red Spheres are to be avoided; touching one results in a GameOver. Star Spheres are bumpers that will send you moving in reverse in the direction you came from until you move forward or make a turn. Yellow Spheres jump you five spaces forward. Rings can be found on the stage as well; collecting all Rings in a stage is required alongside getting all the Blue Spheres for a "PERFECT". Grids of Blue Spheres larger than 3 by 3 cells can be turned into Rings by collecting the Blue Spheres that make up the perimeter of the grid, and ''only'' those spheres.

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 CompilationRerelease locked into the top slot of ''[=S3&K=]''. It is playable in ''VideoGame/SonicJam'' by simulating the lock-on procedure with the ''Sonic & Knuckles'' cartridge in the menu. It also appears as its own game option in ''Sonic Mega Collection'', ''Sonic Mega Collection Plus'' (which gave the minigame an official name for the first time), ''Sonic & Knuckles Collection'' for the PC, and ''Sonic Origins'' (another CompilationRerelease for MediaNotes/TheNinthGenerationOfConsoleVideoGames).

''Blue Sphere'' makes a return in ''VideoGame/SonicMania'' as the Special Stage format for Star Posts. Since the "Catch the UFO!" stages are already used for Chaos Emeralds, ''Blue Sphere'' is instead used for collecting Medallions, which are required for unlocks. This version is [[{{Retraux}} designed to look like its 16-bit counterpart]], though with a few liberties taken to make it look a bit more modernized such as smoother sprite animations and a 16:9 aspect ratio. Additionally, the standalone unlockable version of ''Blue Sphere'', taking cues from a [[https://www.youtube.com/watch?v=cACZxuUy6zY "Blue Spheres 2" proof-of-concept]] created by ''Mania'' co-developer Simon "Stealth" Thomley, adds Green Spheres and Pink Spheres: touching a Green Sphere turns it into Blue Sphere, and touching a Pink Sphere teleports you to a corresponding Pink Sphere elsewhere on the grid.

Finally, included alongside the Retro Engine rebuild of ''Sonic 3 & Knuckles'' created for ''[[CompilationRerelease Sonic Origins]]'' are two different versions of ''Blue Sphere'': the original version returns with Tails being PromotedToPlayable and some of ''Mania''[='=]s improvements (albeit still stuck in 4:3), and new to this release is ''[[https://www.youtube.com/watch?v=8Z0084ocVg0 New Blue Spheres]]'', a realization of Stealth's "Blue Spheres 2" concept, complete with Green Spheres, Pink Spheres, and thousands upon thousands more stages to test your skills at.

[[SimilarlyNamedWorks Not to be confused with]] ''VideoGame/StarOceanBlueSphere''.
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!!NO TROPES! NO TROPES! NO TROPES! NO TROPES?
* AntiFrustrationFeatures: Rings are counted downwards rather than upwards like the ''Sonic 3 & Knuckles'' Special Stages, which makes getting a Perfect easier since you'll always know how many Rings are left on the stage (Perfects are also what are needed to get the Gold Medals [32 of them] when these stages are played as part of Mania Mode in ''VideoGame/SonicMania''). This format of counting down the rings to a Perfect is also what is used for the ''Sonic 3 & Knuckles'' Special Stages in ''Sonic Origins''.
* TheCameo: After completing a stage, the difficulty of that stage is displayed. If the difficulty rating is high enough, sprites of Mecha Sonic, [=EggRobo=], and the animal buddies can appear.
* CastSpeciation: {{Subverted|Trope}}. You can play as Sonic or Knuckles (or even Tails in the ''Origins'' remaster), 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.
* DifficultyByAcceleration: You move faster the longer you take to complete the stage.
* DungeonBypass: Getting every ring in a stage in the full version of the game to get a "Perfect" skips you 10 stages ahead.
* EasterEgg: The game's entire existence. 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.
* EndlessGame: After Level 128016000, 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.
* ExactWords: The ''Sonic & Knuckles'' manual states that the game's lock-on technology won't work with non-''Sonic'' games. While it clearly states that you're only supposed to use ''Sonic 2'' or ''3'' with it, the manual doesn't rule out using ''Sonic 1'' or ''Sonic Classics 3 in 1'' as well.
* InconsistentSpelling: Some sources, including ''Sonic Origins'', list this game as ''Blue Sphere'''s'''''.
* NoPlotNoProblem: Just press the buttons and go right in.
* NoTitle: The original minigame was never given a title. ''Sonic Mega Collection'' was what gave it the title that it is known as today. In ''Sonic & Knuckles Collection'', it was simply called "Special Stage Mode".
* OneHitPointWonder: If you touch a red sphere, you instantly lose.
* PasswordSave: In addition to the password system letting you save your progress through the "full" version of the game, beating the stage given by a non-''Sonic 1'' or ''Sonic Classics 3 in 1'' cartridge earns you the password for that stage.
* PuzzleGame: Collect the blue spheres and rings while avoiding the red spheres.
* SuspiciouslySpecificDenial: 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. This is further emphasized with the opening screen of ''Blue Sphere'' saying, "NO WAY! NO WAY! NO WAY! NO WAY?" Notice the question mark.
* VideoGameGeography: Topologically, the levels are toroids. Visually, they're spheres. The disconnect can be disorienting, if you notice it while playing.
* ViolationOfCommonSense: This is a game you play by going against what the manual tells you to do.