About 15 kilometres south of the city centre, Choeung Ek is the best known of more than 300 killing fields scattered across Cambodia. Between 1975 and 1979 the Khmer Rouge brought roughly 17,000 prisoners here from S-21 and executed them — usually by blows to the head, to save bullets — and buried them in pits across the orchard. Around 9,000 bodies have been exhumed.
The site is a memorial first, a museum second. A 17-storey glass stupa in the centre holds more than 5,000 skulls, arranged by age and sex. The audio guide leads you past the killing tree, the mass graves still slowly surrendering fragments of bone and cloth to the rain, and the small platforms that once held the trucks bringing prisoners. Allow at least two hours, and combine the visit with Tuol Sleng. Tuk-tuks from central Phnom Penh cost around US$15 return.