Hidden history: Radar probes mass graves from Khmer Rouge era A man walks gingerly over a small field in rural Cambodia, pushing a lawnmower-like contraption that deploys ground-penetrating radar to unearth clues of mass graves. The pilot project is twinning technology and fieldwork to locate remains of victims of the Khmer Rouge, the ultra-Maoist regime whose quest to build an agrarian utopia from 1975-1979 left an estimated two million Cambodians dead.