Cambodia: A Tour Bus for visiting Tuol Sleng (S-21) and the Choeung Ek Genocidal Center, Phnom Penh. About 12 km (7?? miles) south of Phnom Penh lie the infamous Killing Fields of Choeung Ek. Here victims of the Khmer Rouge, including many from Tuol Sleng, were taken for execution and burial in mass graves. Many of these have now been exhumed, and a stupa-shaped mausoleum has been erected to their memory.