Sonic the Hedgehog has appeared in more games than we can count, but just as interesting are some of the games that never saw release. Join us as we delve through the Sonic archives to take a look at some of the unfinished and unreleased games in the hedgehog's history.

Sonic the Hedgehog: The Animated Series

Back in the 1990s Sonic was everywhere, with not one but two cartoon series dedicated to his name. While one was a generally light-hearted affair about chilli dogs and slapstick robots, Sonic the Hedgehog: The Animated Series (also known as SatAM) cast the hero as the leader of an underground resistance on the oppressed Planet Mobius.

SEGA Technical Institute in the U.S. began work on a video game based on a cartoon series (based on a video game), doing away with the series' famous speed in favour of a slower, almost stealthy experience: Sonic was able to move back and forth between the foreground and background, throwing rings as weapons and using them for special moves.

The game never got made it past the concept stage, but gameplay elements would later turn up in the cancelled Saturn project Sonic X-treme.

Sonic Crackers

While any new ideas in Sonic the Hedgehog: The Animated Series game were left well behind, Sonic Crackers' main concept went on to form the basis of 32X adventure Knuckles' Chaotix.

Sonic and Tails each hold a ring that exerts a band of energy, holding the two characters together. Using the energy's elastic nature, it's possible to fling your team mate around to reach higher areas, gain speed and more.

Often incorrectly considered to be SEGA's attempt at Sonic the Hedgehog 4, the game was likely an engine test for Knuckles' Chaotix, and although the game was never released a short 'beta' version is widely available online. Interestingly the beta includes overworld "field" stages that presumably were to act as hub levels, a feature that was reintroduced many years later in Sonic Adventure.

Sonic Extreme

Not to be confused with another cancelled Sonic outing, the Saturn project Sonic X-treme, Sonic Extreme was a skateboarding game on Xbox that was unearthed just a few months ago.

Discovered on a developer's Xbox, the game was likely a pitch from developer Vision Scape Software that never got past the approval stage. The videos released so far show a side-by-side battle mode, not unlike GameCube release Sonic Adventure 2: Battle, as well as a trick-based mode that sees Sonic pulling off flips and manuals in a Green Hill-inspired skate park.

Considering Sonic Team's own Sonic Riders was released on the Xbox, GameCube and PS2 it's likely SEGA didn't want two boarding games in the same generation, so another hedgehog outing was consigned to the recycling bin.

Sonic Saturn

One of the most common reasons touted for the Sega Saturn's undeserved death was the lack of a true 32-bit Sonic game. While PlayStation owners tucked into Crash Bandicoot and N64 gamers rejoiced in Super Mario 64, Saturn fans were left wanting: a spruced-up port of isometric Mega Drive outing Sonic 3D, compilation Sonic Jam and racer Sonic R all failed to fill the void of a system-selling Sonic adventure.

It needn't have been this way, though. Sega Technical Institute (STI) had multiple Sonic projects in the works, and although Sonic X-treme is the most well-known it's the untitled Sonic Saturn that arguably showed the most promise.

Although little of the main adventure has been shown, the concept art revealed shows a tantalising glimpse of Sonic chasing Sonic CD nemesis Metal Sonic across a wetlands boardwalk, with the traditional loop-the-loops and giant Robotnik city hiding away in the background.

The game's bonus stage also merits mention: placing Sonic on a pool table, players would use the 'hog's spin dash move to pot the balls and earn a Chaos Emerald. After the game was canned, STI attempted to resurrect the pool bonus stage for Sonic 3D on Saturn, but SEGA went with a 3D version of the iconic Sonic the Hedgehog 2 tube special stage.

Sonic Mars/X-treme

Arguably the most famous unreleased Sonic game took a long and complicated route to incompletion. Another Sega Technical Institute production, the project went through many incarnations, starting off on the Mega Drive before moving to 32X, Saturn and finally PC, never seeing the light of day on any system.

In its earliest form, the game was to be an isometric action game on Mega Drive, long before Travellers' Tales created Sonic 3D, but with the 32X on the horizon SEGA believed the add-on needed a solid Sonic game to lure in the masses. Codenamed Sonic Mars, a reference to the system's codename, the game would tie in with the still-running Saturday morning cartoon show. However, development difficulties stunted the game's progress, and with the 32X dying off SEGA moved the game over to Saturn where it became Sonic X-treme.

The video below is from the animation pitch used to sell SEGA bosses on the idea of Sonic X-treme:

Unlike many of the games in this article that only exist at the concept stage or as ROM dumps, Sonic X-treme was shown off by SEGA itself on more than one occasion, even making an appearance at E3. The game was a very different beast to any Sonic game before it: a fish eye-style camera lens gave the world a warped look and stages would rotate, letting Sonic run up walls and along ceilings, with the gravity changing accordingly.

The game's development could hardly have been more difficult. Project lead Michael Kosaka left the game after a year, with Christian Senn stepping up to the role of producer to try to steer the game back onto course. After a further year's development the project was still experiencing difficulties — while technical work and game concepts were doing well, the game itself wasn't coming together as a whole, and so SEGA brought in outside firm POV to take over technical development.

POV's involvement failed to save the project, though: when SEGA of Japan's then-president Hayao Nakayama paid the studio a visit to check on development, POV's vision so incensed him that he reportedly stormed out of the presentation. However, Nakayama was more keen on a demonstration he had seen of the engine powering the game's boss mode, a NiGHTS-like 3D affair, and instructed STI to make the game in that fashion.

The team behind this new version, codenamed "Project Condor", was under even more pressure than before: Christian Senn claims that team members were moved to a separate building, provided with meals and beds and locked in, with a deadline to finish the game before Christmas 1996. It was never concluded of course, and after over three years of development, platform changes and internal politics, the plug was finally pulled on Sonic X-treme.

The whole sorry affair was enough to kill off Sega Technical Institute and its members disbanded. In recent years, former STI member Christian Senn has released documents, artwork and music from the cancelled project, as well as an in-depth Sonic X-treme FAQ that's a fascinating insight into the most famous Sonic game we never played.