Prison Island

Prison Island (プリズンアイランド) is a location that appears in the anime series Sonic X and its comic series published by Archie Comics. It was a high security military base under the authority of GUN before it was destroyed by Dr. Eggman. Its chief watcher was Howard Watcher.

Past
Prison Island is a considerably old military base, evident by the fact that Gerald Robotnik was imprisoned within said base some 50 years ago. Even five decades later, the equations he drew in his cell remained.

Shadow Saga
Shortly after Perfect Chaos' attack on Station Square, Prison Island was invaded by Dr. Eggman upon discovering the research and works of his grandfather, Prof. Gerald. Inside the prison, Eggman released Shadow the Hedgehog, who informed Eggman of the Eclipse Cannon, saying that with the seven Chaos Emeralds, Eggman could use the canon to rule the world.

Sonic the Hedgehog was later imprisoned on Prison Island by GUN when he was framed as a criminal. No sooner, Tails, Amy, Chris, and Tanaka arrived there to save Sonic. Meanwhile, Eggman, Rouge and Shadow came to the island to steal the Chaos Emeralds kept there. While there, Tails did battle with Eggman and Amy infiltrated the prison, freeing Sonic from his prison. Dashing off into the nearby forests, Sonic encountered and had a quick quarrel with Shadow, before learning that the island was about to be blow up by Eggman's bombs. Running away, Sonic managed to escape the island alongside the rest of his allies just as the island was reduced to a smoking ruin. Meanwhile, Eggman and his associates got away with the Emeralds and Chris.

Archie Comics
After its destruction, the remains of Prison Island were used as a hideout for the shadowy organization S.O.N.I.C.X.