The Graviton Prison is a maximum-security holding facility for the most vile criminals in the galaxy. Prisoners are kept under a heavy-gravity field to limit their movements while giant Warden Robots patrol tirelessly. Even if a prisoner were able to escape, the entire prison sits at the mouth of a black hole, and it is impossible to leave without a ship outside to pick you up.
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Big Convoy's crew came to Graviton Prison in search of an Angolmois capsule but found that the giant Warden Robots had malfunctioned due to the long time spent in heavy gravity and they believed the Maximals were prisoners, locking them up.
Using the Prison's enhanced gravity to his advantage, Big convoy threw himself into the air and gave a mighty stomp, tearing a hole in the floor of his cell. The Maximals escaped and located the capsule while Navi brought the Gung Ho around for a pickup. The giant Warden Robots grabbed the Gung Ho as it attempted to depart, but Big Convoy again kicked them, knocking them off into space- and into the black hole.