An interrogation camp by the Khmer Rouge – a former school known as S-21. Documented 12,000+ (but up to 20,000) people came here and only a few survived (20-200 depending on source).  You can read more here.  There were many of these places during the Khmer Rouge rule.

No photos can do justice to show how horrible this place is.  One of their sayings was “To kill the grass, you have to kill the roots”. That was their justification for killing innocent family members and children of suspected enemies.