Tuol Sleng Genocide Museum

From 1975 until 1979 this former high school was used as a grisly Khmer Rouge prison known as S-21, through which more than 13,000 people passed before being brutally killed. The museum, where visitors can enter the classrooms-turned-cells (some of which still contain the iron shackles used to contain prisoners), is a difficult but important part of any trip to Phnom Penh. The ground floor displays thousands of black-and-white ID photographs taken of victims who passed through S-21.