The Tuol Sleng Genocide Museum (Khmer: សារមន្ទីរឧក្រិដ្ឋកម្មប្រល័យពូជសាសន៍ទួលស្លែង) 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 (Khmer [tuəl slaeŋ]) 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 killed.