The Great Underwater Prison Impel Down 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

Impel Down is located in the middle of the Calm Belt, where Sea Kings thrive, making entrance difficult. The only way to get there with the greatest of ease is to use the Tarai Current, a water current accessible by going through the Gates of Justice located at either the Marine Headquarters or Enies Lobby. Also, the only way to safely get to the prison is to use one of the Seastone lined Marine battleships, as the entire place is surrounded by Sea Kings.

Because of its brutal nature and being renowned for being inescapable since Shiki, almost no person wishes to ever go there. However, there are people who choose to enter it willingly: such as Monkey D. Luffy, Daz Bones and Crocodile.

Usually, Impel Down is a hell-fortress with maximum defenses, making intrusion and breaking-out by criminals impossible. However, Luffy made the record of being the first to break-in, tarnishing that perfect record since Shiki's breakout twenty years ago. The Blackbeard Pirates are the second group of people to ever instigate a break-in, and they did so with less effort than Luffy, not having bothered to try to avoid detection.

Impel Down is a large submerged tower-like structure whose foundation is at the very bottom of the sea bed. Due to it being built within the Calm Belt, the entire structure is constantly surrounded by gigantic Sea Kings swimming below the water. Along with these beasts, the prison is guarded by a force of Marine battleships.

Within the dark confines of the prison are various cells and torture chambers that are used for the prisoners. The cells are all made from seastone to keep those with Devil Fruit powers from escaping. The torture chambers are used for all sorts of brutal unspeakable acts. Among these include continuous whipping, brutal beating to death, boiling cauldrons for burning and drowning, and spiked pits used for impaling.

The prison is carefully monitored all around by a Surveillance Den Den Mushi. Tiny versions of them are found in specific locations throughout Impel Down. Whatever they see is transferred to a larger version that sends video feeds to monitors connected to it, allowing the guards to keep surveillance. Each of the smaller versions has an alarm beeper connected to the shells. The large Surveillance Den Den Mushi is kept in a monitor room somewhere within the fortress.

There is also what appears to be a medical laboratory stationed somewhere within the great jail, with doctors and a variety of medical equipment for immediate treatments of anyone within the prison, both prisoner and staff alike. However, the doctors seem to have a limit, as they cannot produce a cure for an accumulation of various poisons.

The generator room of the entire prison is located on the first floor, before Level 1, and as are the controls to open the Gates of Justice.