Hidden down a peaceful side street bordered with bougain- villea, this memorable, if disturbing, museum was originally a school that was turned into the Khmer Rouge torture headquarters. Tuol Sleng Prison, also known as S-21, was the largest detention center in the country and subjected 17,000 men, women, and children to torture en route to the Killing Fields of Choeung Ek; most did not get that far. When Vietnamese forces liberated Phnom Penh in 1975, they found only seven people still alive at S-21, each having survived because of their skills  as artist or photographer.

The prison has now been converted into a museum; its former cells and gallery are covered with thousands of haunting photographs of subjects before and after torture. There is an interesting exhibition on the second floor of the main building, which gives important details on the  main instigators of the murderous regime, as well as  photos and diaries, and poems written by those affected. The balconies on the upper floors are still enclosed with the wire mesh that prevented prisoners from jumping to an early death. Despite allegedly being haunted, today the place plays host to young footballers on its lawns and a colony of bats in one of the stairwells.