The filthy dungeon was uncovered by Iraqi police after ISIS was defeated in the town of Hammam al-Alil, south of Mosul.
Footage and pictures taken inside the derelict building reveal seven small windowless cells where hundreds of captives were brutalised by torture instruments and killed.
Smashed glass was littered over the floor of the jail which was disguised to look like other houses on the street.
Thick curtains were also placed over the windows to ensure no neighbours or outsiders could look inside the ISIS interrogation room.
Jihadis set fire to the building where they made their escape to destroy the evidence – so it is now just a burnt-out shell.
Riad Ahmad, a resident of Hammam al-Alil, said the terrorist group held hundreds of people in...Video HERE