Prison Island

Prison Island (プリズンアイランド Purizon Airando?) is a location in the Sonic the Hedgehog series. It is a large jungle island used as a prison and research base by G.U.N.. The island had several facilities on the base, including a major port for various military vessels, as well as an armory within the jungle. The main base contained several prisons (hence the name), and also had six levels of security, the highest of which contained Shadow the Hedgehog's cryogenic pod.

Sonic Adventure 2
First seen in Sonic Adventure 2, Dr. Eggman first came there to find G.U.N.'s secret weapon, which turned out to be Shadow the Hedgehog. Some time later, Sonic the Hedgehog was captured by G.U.N. due to a misunderstanding and imprisoned deep inside the island, but he was quickly rescued by Miles "Tails" Prower and Amy Rose. At the same time, Shadow, Eggman and Rouge the Bat attacked the island in order to take G.U.N.'s Chaos Emeralds. They succeeded and planted several explosives on the island, blowing it up as everyone escaped.

Shadow the Hedgehog
However, Shadow the Hedgehog revealed that the explosives had only destroyed G.U.N.'s main base and spilled toxic waste into the rivers. G.U.N. still has some overgrown paths and prison cells in the jungle and they use them to hold captured Black Arms soldiers. In the game, Charmy Bee goes to the island to look for some hidden data disks on behalf of Vector the Crocodile and the player can choose to help him find them.

Sonic X
Prison Island appears in the anime series Sonic X, during the adaption arc of Sonic Adventure 2. The location's role mainly mirrors of that in Sonic Adventure 2.

Trivia

 * Prison Island appeared to be in the shape of an acoustic guitar in episodes of Sonic X . However in other instances, the island has a completely different shape.
 * Numerous prisons in the base bears heavy resemblance to ones in real-life's Alcatraz Island.
 * Knuckles is the only playable character in Sonic Adventure 2 that does not visit Prison Island nor does he play a stage there or the surrounding forest area on the island.