In the mid- to late 1970s thousands of Khmer Rouge prisoners who had been tortured at the infamous Tuol Sleng prison were taken to the Choeung Ek extermination camp for execution. Today the camp, 14 km (9 miles) southwest of downtown Phnom Penh, is a memorial, and the site consists of a monumental glass stupa built in 1989 and filled with 8,000 skulls, which were exhumed from mass graves nearby. It's an extremely disturbing sight: many of the skulls, which are grouped according to age and sex, bear the holes and slices from the blows that killed them. The site is at the end of a rough and dusty road, and can be reached in 30 minutes by motorbike, tuk-tuk ($12 is a reasonable price), or car. The audio tour, available in English, is excellent and well worth the small additional fee.