Tuol Seng is nestled within in the heart of Phnom Penh, the capital of Cambodia. From the outside it looks unremarkable, a multi-story former high school with whitewashed walls and grey shutters. However, inside, Tuol Seng holds a dark secret.

Under the Khmer Rouge Tuol Seng was transformed from a high school into Security prison 21 (S-21), a place in which some of the most violent atrocities of the Pol Pot’s regime were perpetrated.

Originally named Chao Ponhea Yat High School, the complex includes five buildings in total. Several months after the Khemer Rouge took power in 1975 they implemented measures to convert the high school into a high-security facility. Between 1975 and 1979 it is estimated that over fifteen thousand people were imprisoned at Tuol Seng. The prison held between one thousand and fifteen hundred occupants, along with a large security force to keep them in check.

Life within the prison was terrible. Prisoners were shackled into their cells, forbidden to talk to one another and frequently beaten and tortured by the guards. All prisoners were interrogated during their stay at Tuol Seng. These interrogations normally included torture: Prisoners were shocked with electricity, burned with irons and water-boarded.

Today, the former prison stands as a monument to the brutalities of the Khmer Rouge. It is now a museum dedicated to preserving the memory of those who passed through its gates. Rooms that were once used to torture and abuse are now lined with thousands of photographs of the incarcerated. Several of the implements that were used to torture the prisoners are also on display along with a “skull map,” a map encompassing all of Cambodia made entirely from human skulls and bones.

It is said that the ghosts of those who never made it out of Tuol Seng still haunt the building to this day, wandering the corridors and rooms unable to leave the site of their final demise.