This was originally a Chinese cemetery before becoming an execution ground for the Khmer Rouge during their maniacal reign under Pol Pot from 1975 to 1979. Choeung Ek is one of many mass graves all over the country dating from the days of Democratic Kampuchea. This particular site is a collection of innocuous-looking mounds, near a towering monument of cataloged human skulls. The monument is 17 stories high, reminding visitors of April 17, 1975, the day the Khmer Rouge took over Cambodia. As a sign of respect, you take your shoes off before mounting the steps to view the monument up close. Human skulls, arranged by age and gender, are set at eye level, while other bones are placed on higher levels. The Killing Fields are often visited in conjunction with a tour of Tuol Sleng.