Mass burial pits are scattered throughout the grounds of the Choeung Ek Genocidal Center near Phnom Penh, Cambodia. Choeung Ek is the most well-known of the more than 300 mass execution sites now known as the Killing Fields that were used by the Khmer Rouge from 1975 to 1979. An estimated 2 million Cambodians -- or one quarter of the population -- died as a result of the Khmer Rouge's murderous policies, and at least 1.4 million of them are believed to have perished at Choeung Ek and other killing grounds. March 1, 2012.