The Tuol Sleng Genocide Museum is located in the city centre and chronicles the period of the genocide under Pol Pot. The site is a former high school, which was used as the notorious Security Prison 21 (S-21) by the Khmer Rouge regime from its rise to power in 1975 to its fall in 1979.
Tuol Sleng means "Hill of the Poisonous Trees" or "Strychnine Hill". Tuol Sleng was only one of at least 150 execution centers in the country and as many as 20,000 prisoners there were later killed.