Cochlea was built at an unknown date to imprison ghouls who were used for information or materials to make quinque. The One-Eyed Owl attacked the facility and freed many ghouls. Ten years later, Aogiri Tree attacked the detention center and freed several of the imprisoned ghouls, but they were unable to reach beyond the SS level.
Cochlea's defenses were improved after the Aogiri Tree's break-in, to the degree Tatara firmly believed it was impossible to breach again.
Cochlea is a long building that is wide open at the top, which makes it possible to see the sky inside. While the path is only at the surroundings near ghouls' isolated jails, the center is a deep fall. The structure's walls are coated with quinque steel to help keep ghouls secured within.
The building is divided into many deep floors:
- First Floor
- Designated floor to imprison the lower-rated ghouls.
- Second Floor
- Designated floor to imprison the S-rated ghouls.
- Third Floor
- Designated floor to imprison the SS-rated ghouls.
- Fourth Floor
- Designated floor to imprison the SSS-rated ghouls, also the deepest known level in Cochlea.
- Disposal Room
- The level it is located at is unknown; fittingly named as it is where ghouls are sent through a compactor to be crushed to death, thereof "disposed."
- Shinme Haisaki † - Warden (Formerly)
- Kyouji Misaka † - Warden (Formerly)
- Goumasa Tokage † - Interrogator (Formerly)
- Shiki Kijima † - Interrogator (Formerly)
- Akashi Kobayashi - B+ rated
- Donato Porpora † - SS-rated (Escaped)
- Matasaka Kamishiro - SS-rated (Escaped)
- Roma Hoito † - SS-rated (Escaped)
- Rio - SS-rated (Escaped)
- Unnamed SS inmate - (Escaped)
- Ken Kaneki - SS-rated (Released)
- Hinami Fueguchi - SS-rated (Escaped)
- Eto Yoshimura - SSS-rated (Escaped)
- Yuuma † - Unknown rating
- :re Volume 4 includes a diary kept by Hinami during her imprisonment. It is revealed that the prisoners actually have some privileges and rights, such as designated exercise times and the ability to interact with other prisoners.
- The prisoners are given something described as a "stew", though it is unknown what it is made from or where the ingredients come from.
- Information on the location of each prisoner is highly restricted and only accessible to Prison Staff. Even senior ghoul investigators cannot access this information.
- The name Cochlea is a reference to the primary organ of hearing in the human ear of the same name. The cochlea is coiled like a funnel from wide on top and narrowing downward in levels to process sound.