The Tuol Sleng Genocide Museum is a museum in Phnom Penh, the capital of Cambodia. 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 in Khmer; [tuəl slaeŋ] means "Hill of the Poisonous Trees" or "Strychnine Hill".