Blue Sphere (ブルースフィア Burū Sufia?) is an easter egg video game built into the Sonic & Knuckles cartridge for the Sega Mega Drive, accessible by attaching any Mega Drive game cartridge (other than Sonic the Hedgehog 2 or Sonic the Hedgehog 3) into the Sonic & Knuckles lock-on port. Blue Sphere is a collection of Special Stages in the same style and engine of those featured in Sonic the Hedgehog 3 and Sonic & Knuckles.
Accessing Blue Sphere
When a Mega Drive game cartridge other than Sonic the Hedgehog 2 or Sonic the Hedgehog 3 is locked on to the Sonic & Knuckles special cartridge, a screen appears with the Sonic & Knuckles logo, models of the characters Sonic, Tails, Knuckles and Eggman, and a line of scrolling text that says "No Way! No Way! No Way! No Way?" While this screen is visible, pressing , and at the same time changes the screen to the Blue Sphere menu. The scrolling text will be replaced by "Get Blue Spheres!"
If the cartridge that is locked on to Sonic & Knuckles is the original Sonic the Hedgehog (Sonic Classics, the Nick Arcade and Simon Wai prototype versions of Sonic the Hedgehog 2 will also work), then the full game of Blue Sphere is accessed with all stages. Most other cartridges will allow you to access only a single stage of the game, based on the cartridge's ROM header and the stage's code in the full Blue Sphere game. Some cartridges with particularly large ROM file sizes (including Phantasy Star IV: The End of the Millennium and Super Street Fighter II: The New Challengers) will not load Blue Sphere, but will instead boot Sonic & Knuckles as normal.
Blue Sphere is also available in certain compilation games. In Sonic Jam, it can be accessed by "locking-on" Sonic 1 to Sonic & Knuckles. In Sonic Mega Collection and Sonic Mega Collection Plus, Blue Sphere can be unlocked separately (in the former case by playing Sonic the Hedgehog and Sonic 3D Blast 20 times each) and is accessible by itself from the game listing. It can also be accessed from the file menu's special stage selection in Sonic & Knuckles Collection for PC.
The main menu of Blue Sphere will show the current level number along with a 12-digit code (password) that will allow the player to jump back to the current level. Pressing , , or will switch the playable character between Sonic (represented by blue-star bumpers either side of the word 'START') and Knuckles (represented by red-star bumper). (The player cannot play as Tails, or as Sonic and Tails together.)
Blue Sphere levels are based on the Special Stages from Sonic the Hedgehog 3 and Sonic & Knuckles. The player must run around the surface of a 32x32 wrapping square (torus), collecting blue spheres while avoiding red spheres. Blue spheres, once collected, turn red. Silver bumpers with red stars bounce the player, causing them to run backwards for a short time (until the player pushes forward on the D-pad), and orange spheres catapult the player a certain number of squares forward. Collecting all blue spheres will award the player with a Chaos Emerald and (if the full game has been accessed) progress to the next level.
In addition, the stages will contain a set number of rings to collect. Some rings will be already visible in the stage, others will be hidden. These rings can be revealed by surrounding blocks of blue spheres completely by an outline of red spheres—the entire block will then be turned into rings.
There is a slight difference between these stages and those in Sonic the Hedgehog 3 and Sonic & Knuckles; in Blue Sphere, the ring counter counts down instead of up. If a level is completed with all of the rings collected, the player receives a "perfect" grade and advances ten levels instead of just one.
Blue Sphere has 134,217,728 (227) levels, which are played in sequence. Once a player reaches the final level, the sequence repeats from the beginning. (Note, as mentioned above, collecting every ring and scoring a perfect allows players to advance ten levels.)
In fact, there are actually only 128,016,000 distinct level layouts (or stages). After 128,016,000, the stages repeat, even though the level number continues to advance. After level 134,217,728, however, the level number resets too.
These levels are not unique in their design. Each level is a 32x32 wrapping square (torus) that consists of four 16x16 segments. The layout of each segment is determined from a master list of 128 set layouts (numbered 0 to 127) based on a linear congruence using the level number. If the level number is N, then the top-right segment is (N-1)%128, the bottom-right is (3N-2)%127, the top-left is (5N-3)%126, and the bottom-left is (7N-4)%125, where %n means remainder after division by n. This formula produces the discrepancy between the 227 levels and the 128,016,000 (the least common multiple of 125, 126, 127, and 128) stages.
Each stage has a difficulty, based on the number of spheres in each region of the stage's map. The difficulty is displayed each time the player completes a level, and is represented by a collection of sprites that appear on screen. For each additional difficulty level, an extra sprite is added to the screen (or, in the case of MAX difficulty, a sprite is changed). There are thirteen difficulties. The sprites that appear on screen are as follows:
- Mecha Sonic
- EggRobo in an Egg Mobile
- Four chickens (Cucky)
- Four squirrels (Ricky)
- Four rabbits (Pocky)
- Master Emerald
- Sonic & Knuckles logo
- (MAX) Sonic sprite turns into Super Sonic
There are two 12-digit passwords for each level, one for the version from which the player can advance (the full game, which can be accessed by locking on Sonic the Hedgehog) and one for the version from which they can not (the levels accessed by locking on other cartridges). These codes can be input to the full game. By inputting a code from the full game, the player can skip to the relevant level and continue from there; by inputting a code from the non-advanceable game, the player can play that level, but completing the level will not let the player advance further.
Compilation game releases
- Sonic & Knuckles Collection, PC (1997)
- Sonic Jam, Sega Saturn (1997)
- Sonic Action 4 Pack, PC (2001)
- Sonic Mega Collection, GameCube (2002)
- Sonic Mega Collection Plus, PlayStation 2 and Xbox (2004)
- Sonic PC Collection, PC (2009)
- Virtual Console, Wii (2010)
- Blue Sphere Madness by ApachaiHapachai at YouTube