Location: Phnom Penh, Cambodia
Notable Residents: n/a
Amenities: Rooms covered with black and white photos of former prisoners

The Tuol Sleng Genocide Museum, or Security Prison 21 as it was known during the Khmer Rouge regime from 1975 to 1979, was a high school before it was transformed into one of the most frightening prisons in the world. Prisoners were routinely tortured to force confessions for crimes, after which the prisoners and any co-conspirators were promptly executed.

The prison was finally discovered by a Vietnamese photographer who, along with colleagues, followed the stench of rotting corpses to Tuol Sleng. Today, the prison is a museum featuring rooms lined floor to ceiling with black-and-white photos of the 17,000 estimated prisoners who were held there. The museum is best known for its "skull map"—a map of Cambodia made with 300 skulls and other bones found by the Vietnamese when they occupied the country. It served as a reminder of what happened at Tuol Sleng until it was dismantled in 2002, but some of the skulls are still displayed on shelves in the museum.