From the outside, the burnt-out shell of a building appears like any other derelict house.
But the debris left inside sheds light on the atrocities that took place inside its walls.
The prison was discovered by Iraqi police after ISIS were defeated in the town of Hammam al-Alil , 10 kilometers south of Mosul.
Residents said a one-square-metre cell once held as many as four prisoners and that people were hanged in the staircase.
Iraqi police said that ISIS made the second-floor living room an interrogation room concealed from the street outside.
Witnesses said that jihadis set fire to the building when leaving to destroy the evidence and attempt to hide their crimes.
Riad Ahmad, a resident of Hammam al-Alil, said that he remembered hundreds of people taken into the prison by ISIS.
He said that those linked to the Iraqi forces, police or army, were killed immediately, so were those with links outside the area who were seen as spies that could pass on information about Islamic State locations.
He said that ISIS committed many crimes while in control of the town and treated them as fun.
When government forces began to retake the town, Islamic State placed landmines in the surrounding farmland, which have yet to be cleared, Ahmad added.