Security prison 21 in Phnom Penh, Cambodia, otherwise known as Tuol Sleng Museum, was the home of unspeakable horrors. In 3 years, 8 months and 20 days in the late 1970s the Khmer Rouge killed over 2 million of their own people from a population of 6 million. This prison used to be a school and was taken over by Pol Pot's regime to torture and kill. The museum contains photos of the victims and it's a chilling and haunting place.