Impel Down

The Great Underwater Prison Impel Down (海底大監獄インペルダウン Kaitei Daikangoku Inperu Daun?) is the World Government's maximum-security prison for the most dangerous criminals and  pirates. It is located underwater in the middle of the Calm Belt.

History
Nothing is really known about Impel Down at this point, besides that "Thunderscar" Nitro V. Kaian somehow escaped from the Marines while being transported there, dodging the fate many pirates have fallen into.