Central Great Underground Prison

The is an underground prison where Shinigami who broke the laws of Soul Society are sentenced to.

Overview
The Central Great Underground Prison consists of eight levels, in which the Central 46 decides which the criminal Shinigami would be confined, depending on the severity of his or her crimes. The prison is located underneath the barracks of the First Division.

3rd Level: Shugo
is the third level of the underground prison and reserved for those who used forbidden Kidō techniques. Tessai Tsukabishi was to be given a life sentence in this level for using the forbidden Kidō spells Jikanteishi and Kūkanten'i.

8th Level: Muken
is the eighth and final level of the underground prison. Sōsuke Aizen was sentenced to 18,800 years in this level for high acts of treason and a long list of other crimes. His outburst during his trial raised the sentencing to 20,000 years.

Muken is a voided space completely sealed off from the outside world, aside from the main tower that serves as the entrance and exit. The realm stretches on infinitely. Under normal circumstances, non-criminals are not permitted entrance, but Retsu Unohana managed to gain permission from Captain-Commander Shunsui Kyōraku to use the location as a battlefield to train Kenpachi Zaraki in Zanjutsu. Muken is reserved for criminals whose heart cannot be stopped by any means by executioners (i.e. those who are immortal).

When Kyōraku went to visit Aizen in request for his assistance against the Wandenreich, he had the key to the Muken sealed into his chest. Should he be killed, the gates to the outside world would be sealed off, preventing Aizen's escape. Aizen himself is bound by seals in the form of black straps, which could be undone by 19 keyholes on the ground; Kyōraku was given permission to use up to three keys.