Central Great Underground Prison

"As the name "nothingness" implies, this place is void of any contact with the outside world. It also stretches on for infinity. Thus, we decided that it was the only place where we could freely, and without inhibition, cross blades... Otherwise, we who aren't criminals, would never be given permission to stand foot on these grounds."

- Yachiru Unohana describing the eighth level, Muken, to Kenpachi Zaraki.

Central Great Underground Prison (真央地下大監獄, Shin'ōchikadaikangoku) is the correctional facility of Soul Society, where Central 46 sentences those horrendous criminals that have committed treason against the laws of Soul Society. Under special circumstances, the Captain-Commander is allowed to open up the prison for purposes other than imprisonment, such as the training of Kenpachi Zaraki to be overseen by Yachiru Unohana.

Overview
Soul Society's central prison is a vast network of underground chambers all throughout Seireitei, with most of the "levels" hidden below specific Division barracks, with that particular Division being tasked with the monitoring of that prison level. Hence, it is for this reason that the bottom level, Muken, is under the Captain-Commander's jurisdiction, as it is hidden below the First Division barracks. Likewise, the third level, Shugo, which is tasked with handling abusers of Kidō, is under the Kidō Corps' jurisdiction.

Prison Levels
The prison has eight levels which prisoners are confined in based on the severity of their crimes.


 * Ujimushi no Su (蛆虫の巣, "Nest of Maggots"): The first level, it is an underground prison under the supervision of the Onmitsukidō's third corps (known as the Detention Unit). Located in the north-west section of the Second Division's grounds, it is surrounded by a thirty ken (approx. 54.54m) wide moat and a high wall. It is accessible by a wooden bridge. On the inside, it resembles a cave with an area for prisoners. Treatment for the prisoners is mostly lenient; they can walk freely and converse with one another, but the bleak setting drives many to madness. Prisoners are not held inside a cell and are free to roam the Nest, but they may not set foot outside of the facility. The only exception is the Nest of Maggots' former resident Mayuri Kurotsuchi, who was chained inside a cell. When a Shinigami becomes a potential danger to other Shinigami and Soul Society, that Shinigami is brought to the Nest of Maggots for imprisonment, regardless if they were put on trial; this is to ensure no danger will be brought to Soul Society. Neither the staff of the Detention Unit nor any other visitors are permitted to carry a Zanpakutō into the Nest of Maggots. This measure means that the wardens are required to be able to control the prisoners with bare hands. If a Shinigami desires to enter and is not adept at hand-to-hand combat, they may not enter. This is to prevent rebellions and escapes from the prison. Kisuke Urahara, as Corps Commander of the Detention Unit, was once in charge of the Nest of Maggots before being promoted to Captain of the 12th Division.


 * Second Level: There is nothing known about the second level of the Central Great Underground Prison.


 * Shugo (しゅご, "Safeguard"): The third level of the underground prison, it is hidden underneath Kidō Corps headquarters. Nothing is known about it, other than it is meant purposefully to imprison those that have misused Kidō. It is an environment where the molding of reiryoku is rendered impossible, therefore making it an impossibility to cast any form of Kidō spells while in the prison.


 * Fourth Level: There is nothing known about the fourth level of the Central Great Underground Prison.


 * Fifth Level: There is nothing known about the fifth level of the Central Great Underground Prison.


 * Sixth Level: There is nothing known about the sixth level of the Central Great Underground Prison.


 * Seventh Level: There is nothing known about the seventh level of the Central Great Underground Prison.


 * Muken (無間, Voided Space): The eighth and final level of the underground prison underneath the First Division barracks. There is very little known about the Muken, other than what Unohana has described of it. Its name, meaning "nothingness", implies it to be an infinitely-expanding space with absolutely no contact to the outside world. Permission to set foot, other than having been sentenced here by Central 46, can be given only by the Captain-Commander of the Gotei 13. There are special mechanics that deal with opening and sealing the prison, which take a considerable amount of time and effort.

Notable Sentencing

 * Kenpachi Azashiro - sentenced to the eighth level, Muken, for treason against the Soul Society.
 * Tessai Tsukabishi - sentenced to the third level, Shugo, for the use of Forbidden Kidō spells.
 * Sōsuke Aizen - sentenced to the eighth level, Muken, for rebelling against Soul Society and seeking to kill the King of Souls.