Prison Island (プリズンアイランド, Purizon Airando) is a large acoustic guitar shaped jungle island used as a prison and research base by G.U.N.. In the Shadow Saga, Dr. Eggman first came there to find GUN's secret weapon, which turned out to be Shadow the Hedgehog. Some time later, Sonic the Hedgehog was captured by GUN due to a misunderstanding and imprisoned deep inside the island, but he was quickly rescued by Tails the Fox and Amy the Hedgehog. At the same time Shadow, Eggman and Rouge the Bat attacked the island in order to take GUN's Chaos Emeralds. They succeeded and planted several explosives on the island, blowing it up as everyone escaped.
However, in the Black Arms Saga revealed that the explosives had only destroyed GUN's main base and spilled toxic waste into the rivers. GUN still has some overgrown paths and prison cells in the jungle, and they use them to hold captured Black Arms soldiers. Charmy the Bee goes to the island to look for some hidden data disks on behalf of Vector the Crocodile.