Impel Down

Impel Down, both known as the Great Prison (大監獄, Dai Kangoku) and the Underwater Prison (海底監獄, Kaitei Kangoku), is the world's greatest stronghold for the most dangerous of criminals and pirates. It is located somewhere in the middle of the Calm Belt adjacent to Paradise.

Affiliated with the World Government and the Marines, by extension, it houses many of the captured outlaws, framed by those organizations.