Impel Down is one of the most vital places within the One Piece world. Situated right at the center of the Calm Belt, it acts as the World Government's highest-security prison to house the most notorious criminals and pirates. It is officially referred to as Impel Down, but many times, in Japanese, it is more commonly referred to as Kaitei Kangoku, literally "Underwater Prison," or Dai Kangoku, "Great Prison.".
What is unique about Impel Down is its structure and intricate design, comprising six levels that descend deeper into the ocean. With each level, there is a subtle degree of increase in both punishment and security. Its design has been equated to the levels of Hell from Dante's Inferno, where every level is a distinct form of torment for the prisoners.
The deepest, or lowest, level is known as Crimson Hell and is the farthest from the surface. Because this forest of red trees with leaves in the form of blades is designed to retard the progress of any prisoners trying to escape. The second level is referred to as "Wild Beast Hell," and various beasts therein chase and assault the inmates.
Level 3 is called the Starvation Hell, and here, the inmates undergo punishment in very hot conditions with denial of food. Level 4, known as the "Blazing Hell," is a sea of boiling blood and blazing fire. Level 5, called the "Freezing Hell," is one of very cold temperatures; the prisoners are put in freezing conditions for a long period. Level 6, or Eternal Hell, is the very final level and the most secure, reserved only for the most feared criminals the world has ever known, many of whom have been stricken from history.
Impel Down was constructed to house people with Devil Fruit powers. Indeed, the prison is designed using sea stone in order to nullify the powers of the Prisoners of Hierarchy. Indeed, it is one very key fact toward understanding how effective a prison really could be against some of the strongest, most powerful characters in the One Piece universe.
The Impel Down Arc takes place mainly in the prison and forms a large part of the One Piece storyline. The twenty-first story arc of the manga and anime series, and the third of the Summit War Saga: this arc details Monkey D. Luffy trying to enter the prison with the aim to rescue his brother, Portgas D. Ace. During this arc, Luffy meets quite a few characters from his past who will become integral to the events that follow. Among those characters are Buggy the Clown, Mr. 2 Bon Clay, and Mr. 3.
What is very controversial in the One Piece plot is Luffy masterminding a prison breakout. While it has shown what Luffy would do to save his brother, it also opens up some moral problems regarding the Aftermath of his actions. On a larger scale, what Luffy did was inexcusable; so many dangerous criminals were allowed to break out of prison.
Impel Down houses many of the strongest and most infamous pirates in the One Piece world. Among these famous inmates were Portgas D. Ace, Jinbe, Crocodile, and Emporio Ivankov. The presence of these powerful characters within the prison makes it clear how important it is for the story of One Piece but also how crucial it is to the World Government in maintaining their balance of power.
The Impel Down Arc is visually depicted through Season 13 of the One Piece anime series, thus allowing the viewer to see the deplorable conditions of the prison and the very fierce battles that ensued during the prison break. The anime adaptation brings out clearly the different characteristics that define each level, from the blood-red forests on Level 1 to the frozen wastes on Level 5. This, in turn, strengthens in the viewer's mind the understanding of the prison's strictness and mercilessness.