Tuol Sleng is a former secondary school nestled in the heart of Phnom Penh. Code-named S-21, it has served as high security prison and interrogation center during the repressive Khmer Rouge regime. The walls in this picture still bear the marks of Cambodia’ s painful past on their surface, standing as a witness against the darkest side of humanity. They are talking to us and we need to listen carefully, making sure their legacy will not get lost.