Tuol Sleng Genocide Museum occupies the grounds of what was once Tuol Svay Pray High School, a cluster of concrete buildings in central Phnom Penh that the Khmer Rouge repurposed into Security Prison 21 — known as S-21 — between 1975 and 1979. An estimated 17,000 people were imprisoned and tortured within its classrooms-turned-cells during those four years; fewer than a dozen are believed to have survived. Today the site preserves thousands of identity photographs taken of prisoners upon arrival, rows of metal bed frames used during interrogations, and the rudimentary cells built inside former classrooms. Visitors move through galleries of those haunting portraits, confronting what Gates describes in the episode as people who "knew, when the photo was taken, that they were gonna be executed." Gates visited the museum during his first season while investigating Cambodia's broader history ahead of his search for the lost Khmer city of Mahendraparvata, using S-21 as essential context for understanding the civilization that had been nearly erased in living memory.
The site functions as Tuol Svay Pray High School, educating students in Phnom Penh.
The Khmer Rouge seizes power and converts the school into Security Prison 21 (S-21), beginning a campaign of mass detention, torture, and execution.
Vietnamese forces enter Phnom Penh; the Khmer Rouge regime collapses and S-21 is abandoned. Survivors and investigators find evidence of the atrocities committed there.
The site is opened to the public as the Tuol Sleng Genocide Museum, preserving it as a memorial and documentation center.
Gates visits the museum during filming of Expedition Unknown S01E02 "Temple of Doom," reflecting on the genocide before heading into the Cambodian jungle.
Tuol Sleng Genocide Museum stands as one of the most sobering documentary records of twentieth-century atrocity. The Khmer Rouge regime, which ruled Cambodia from 1975 to 1979 under Pol Pot, systematically used S-21 as an interrogation and execution facility. Prisoners — including intellectuals, ethnic minorities, Buddhist monks, and even cadres of the regime itself — were photographed, tortured into false confessions, and then transported to execution sites, most notably the Killing Fields of Choeung Ek on the outskirts of Phnom Penh.
The photographic archive at Tuol Sleng is considered an extraordinary — and deeply disturbing — historical resource. Thousands of identity photographs taken by Khmer Rouge guards have been preserved and studied by historians and human rights investigators, helping to document individual victims in a genocide that claimed an estimated 1.5 to 2 million lives — roughly a quarter of Cambodia's population at the time. Historians and legal scholars have drawn on the S-21 records in proceedings before the Extraordinary Chambers in the Courts of Cambodia (ECCC), the tribunal established to prosecute surviving Khmer Rouge leaders.
Within mainstream scholarship, debate continues around questions of command responsibility, the precise death toll, and the extent to which the genocide was ideologically versus opportunistically driven. The museum itself has been subject to ongoing curatorial discussions about how to present evidence of trauma without exploiting victims — a tension that institutions preserving genocide records around the world continue to navigate.
Gates did not conduct a forensic or archaeological investigation at Tuol Sleng; his visit in S01E02 was journalistic and reflective, offering viewers human context before he pursued the ancient Khmer city of Mahendraparvata. His restrained on-camera reaction — offering an apology to the viewer mid-sentence, then falling silent — models the kind of respectful witness the site demands. The episode does not attempt to resolve historical debates; it simply insists that understanding Cambodia's present requires reckoning with its recent past.
The site was a functioning secondary school before the Khmer Rouge takeover — the transition from classroom to prison cell is visible in the building's architecture, which the regime modified with crude barbed wire and brick subdivisions.
Fewer than a dozen of the estimated 17,000 people imprisoned at S-21 are believed to have survived — a mortality rate that historians describe as among the highest of any documented detention facility of the twentieth century.
The Khmer Rouge kept meticulous photographic records of prisoners upon arrival; those photographs, preserved at the museum, have become primary evidence in both historical research and international tribunal proceedings.
The name 'Tuol Sleng' is believed to translate roughly as 'Hill of the Poisonous Trees' or 'Strychnine Hill' in Khmer, though the precise etymology is subject to some variation in local accounts.
Tuol Sleng Genocide Museum is generally open to the public in Phnom Penh and is considered one of the city's most significant memorial sites; visitors should check current opening hours and admission fees before planning a visit. The museum contains graphic imagery and preserved evidence of torture and execution — it is recommended for mature visitors and approached with appropriate solemnity.
Phnom Penh, Cambodia — the museum is located within the city itself, approximately 3 kilometers south of the Royal Palace.
Cambodia's dry season, roughly November through April, is generally considered the most comfortable time to visit Phnom Penh, with lower humidity and reduced rainfall. The museum can be visited year-round, though mornings tend to be less crowded.
Cambodia National Museum, Phnom Penh
The Cambodia National Museum in Phnom Penh appears in the same season of Expedition Unknown and shares the broader Cambodian cultural and historical context Gates explored in S01E02.
Guatemala Snake King Archaeological Sites
Gates' investigation of the lost Khmer city of Mahendraparvata — the main subject of S01E02 — parallels his work at Maya Snake King sites in Guatemala, both involving rediscovered ancient civilizations whose legacies were nearly lost to time and conflict.
Kingdom of Kush Pyramid Tombs
Like Tuol Sleng, the Kingdom of Kush Pyramid Tombs represent a civilization whose history was marginalized or overlooked, and whose physical remains Gates investigated as a way of recovering suppressed historical narratives.