Skulls are displayed at the Choeung Ek killing fields memorial in Phnom Penh on July 2, 2015. Two former Khmer Rouge leaders July 2 began appeal hearings against their landmark convictions for crimes against humanity last year which saw them handed life sentences by Cambodia's UN-backed court. AFP PHOTO / TANG CHHIN SOTHY (Photo credit should read TANG CHHIN SOTHY/AFP/Getty Images)