After visiting Tuol Sleng Genocide Museum, you should go and see the Killing Fields at Choeung Ek. On the doorstep of Phnom Penh, S-21 prisoners were killed and buried in a communal grave by the Khmer Rouge. An ossuary was built to house the skulls of the 8,000 bodies that were unearthed in the former extermination camp. The audio-guide given out for your visit was made by the survivors of the genocide.