Toul Sleng Genocide Museum (S-21)
Toul Sleng was a High School until the Khmer Rouge took it over in 1975 and used it as a prison and interrogation facility. There are four buildings, each with three floors. Each building and floor was separated by what it was used for. Some floors had small pens where prisoners stayed, others had torture rooms, etc. It is by far the saddest, most disturbing place I have been to yet. Prisoners were systematically interrogated, tortured, forced to write false confessions, and eventually executed. Out of 17,000 prisoners that were kept here, only 12 survived. Much of the buildings have been left in they state in which they were abandoned by the Khmer Rouge in 1974. There are many books on the subject. "Voices from S-21" is a popular one.
Choeung Ek Genocidal Center
This center is a memorial and another one of the many killing fields in Cambodia. This killing field is where most people died who suffered at S-21. There are groups of mass graves. The picture to the left is a memorial stupa that contains thousands of skulls.