Located in Phnom Penh, capital of Cambodia and known as the S-21 prison, Tuol Sleng Genocide Museum was a nightmare for around 20,000 people who were imprisoned, tortured and killed here by the Khmer Rouge regime from 1975-1979. During your visit in Phnom Penh, Tuol Sleng Genocide Museum should be a must-see place to learn about the painful chapter in Cambodian history.