Central Great Underground Prison

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

Overview
The Central Great Underground Prison consists of eight levels, in which the Central 46 decides which would the criminal Shinigami be confined to dependent on the severity of his or her crimes. The prison is located underneath the barracks of the 1st Division.

3rd Level: Shugo
is the third level of the underground prison. Tessai Tsukabishi was to be sentenced to life 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 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 where it is completely sealed off from the outside world, aside from the main tower that serves as the entrance/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.

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.