The Space Colony ARK is a now-abandoned space colony in orbit around Sonic's world. Fifty years ago, it was the long-time home of Professor Gerald Robotnik and his grand-daughter Maria, as well as being the birthplace of Shadow the Hedgehog. The ARK features prominently in both Sonic Adventure 2 and Shadow the Hedgehog.
The design and construction of the ARK - the first "Bernal sphere space colony" (mistakenly called a Bernoulli sphere by Tails) - began over five decades before Sonic's time, and was overseen by Professor Gerald Robotnik. A giant, hemispherical orbital habitat, the ARK hosted a number of researchers and military personnel - along with some of their children, like Maria Robotnik and the young GUN Commander. While obviously a fantastic astronomical engineer, Gerald was later contracted by the President of the United Federation to deploy his biomedical expertise on board the ARK, as the leader a research project investigating the prospects of clinical immortality. Project Shadow, as it was known, produced a plethora of wonderous results, as did Gerald's other projects on the space station, including his cryptoarchaelogical work on the ancient robot Emerl, and his energy weapon designs for the Eclipse Cannon.
In spite of the research's auspicious beginnings, a series of serious accidents on the ARK began to erode the confidence of the planetside military - the Guardian Units of Nations - in the advisability of continuing such boundary-shattering science. The Artificial Chaos rebelled, witnessed by Shadow in The Doom; Emerl ended up devastating sections of the facility after being introduced to military-grade weaponry, as recorded in Gerald's Journal; and Biolizard proved almost uncontrollably ferocious to the scientists who created it. These alarming developments eventually culminated in the GUN shutting down all activity on the station - violently. Using the cover story of a biotoxic spill, GUN evacuated the ARK's civilian population, murdered many of the Project Shadow researchers (and Maria Robotnik), sealed Biolizard in the core, and set Gerald up as a scapegoat, imprisoning and eventually executing him.
Shadow the Hedgehog, Dr. Eggman and Rouge the Bat plan to use the ARK to destroy Earth in Sonic Adventure 2; Eggman as a threat, Rouge was merely their assistant and Shadow wanted revenge. They almost succeeded, but the Eclipse Cannon was damaged and instead, the Chaos Emeralds activated an embedded sequence Gerald had put into the colony where the ARK would destroy Earth and shatter it to pieces to kill all those who had brought pain to his life. Sonic the Hedgehog, Miles Prower, Knuckles the Echidna, Eggman and Rouge all work together to stop ARK, Tails and Eggman go first and weaken ARK's defenses. Rouge drains the water from the Eclipse Cannon (making it less of a threat), Knuckles opens the door, and Sonic and Knuckles get inside the Eclipse Cannon. However, doing those requirements awakens the guardian of the ARK, Biolizard. Luckily, Shadow managed to distract Biolizard long enough for Sonic and Knuckles to get to the Master Emerald shrine. Knuckles speaks the spell to control the Chaos Emeralds and stops them from pulling ARK to the Earth. Biolizard doesn't give up yet and uses Chaos Control to merge with the colony and become Finalhazard. Sonic and Shadow use the Chaos Emeralds and transform into Super Sonic and Super Shadow. Shadow kills the Finalhazard, and the two perform an immense Chaos Control to teleport the ARK back into orbit.
In Shadow the Hedgehog, in the Pure Evil storyline, Shadow and Black Doom use the Eclipse Cannon to destroy Central City. After destroying the guard robot Diablon in GUN Fortress and injuring Sonic, Shadow declares "I'm going to destroy this damn planet!" It is possible that in that non-canon storyline, Shadow went back to ARK and destroyed Earth. In the Hero storyline, Sonic, Tails, Knuckles, Amy Rose, Rouge and Shadow go to ARK to drive out the Black Arms. Also there are Espio the Chameleon, Charmy Bee and Vector the Crocodile. In the Action Stage Cosmic Fall, you have a choice of getting to shelter or helping Vector find the computer room. There are also two Action Stages in the past set on the ARK, The Doom and Lost Impact. Lost Impact is Shadow remembering when he destroyed the Artificial Chaos, and The Doom is the classic sequence of Shadow helping Maria evacuate from the GUN raid.
Cosmic Fall allows Shadow to fight one of two possible bosses in the ARK's computer core: either Black Doom, who is incensed at Shadow's intention to atone for his crimes by surrendering to the humans; or Dr. Eggman in the Egg Dealer, as Shadow proves his determination to safeguard Gerald's last, greatest creation from anyone who would try to malign it.