There’s a ghostly underlay to Phnom Penh, a city beneath the city today. That city is empty. When the Khmer Rouge defeated the US-backed Lon Nol regime in 1975 and took power, their first act was to completely evacuate the capital. The city’s inhabitants were rounded up and put to work in forced labour camps in the countryside. The Khmer Rouge idealised the peasantry (or “base people” in their own parlance): their method for eliminating class distinctions was to make everyone a peasant, to put everyone to work in the fields. For a period of almost five years, Phnom Penh (and other large towns throughout the country) stood deserted. It’s curious to walk through a place, knowing that its human population could disappear so completely: it makes the human presence there seem only provisional. The crowds of tuk-tuk drivers sleeping through the midday heat, the fruit sellers in the markets, the parents queued up on motorbikes to pick their kids up from school: all of these people (or their parents or grandparents) were swept from the place and returned only after terrible privation.

This aura is especially strong around Tuol Sleng (or S-21), the prison where people the Khmer Rouge had identified as enemies were brought for interrogation and torture. As we walked there, I tried to imagine how the prisoners felt, blindfolded, driven through the silent city. The prison’s existence was a closely guarded secret, and so no one sent there was allowed to survive: they would die at the prison or later, at the Killing Fields on Phnom Penh’s outskirts. The city crowds around the prison now: apartment buildings look down into it, and there are restaurants to serve the many people who visit it. Apparently one of these restaurants had a Khmer Rouge theme – with waiters dressed in the cadre’s black uniform, serving the same gruel that the prisoners ate – before the government shut it down.

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Tuol Sleng was a high school before the Khmer Rouge re-purposed it. You can still see the school in the place – the three-storey concrete buildings arranged around a central courtyard, the classrooms running off long balconies; the yellow and white tiled floors; the blackboards. In one of the buildings the cells have been left as they were: on the ground floor, crude walls of brick divide the rooms into cells; the upper floors indicate more forethought, with cells made of panelled wood. One of the things that make the place so chilling is the bureaucratic thoroughness with which it was run. Each prisoner was photographed when he or she arrived, and a file prepared on their background; it was the existence of these detailed records that allowed the prosecution of the prison’s chief interrogator many years later. Many of the rooms now are given over to these photographs: they’re overwhelming in their uniformity of pose (the chair in which the photos were taken is also displayed; a vice is mounted on the back to hold people’s heads straight), their variety of expression (some are openly terrified, others defiant; some have a placid, helpful expression, as if by co-operating with the authorities they might hope for release), and their sheer numbers. The women all have the haircut mandated by the Khmer Rouge: cut short, parted in the middle. The photos of children are perhaps the most upsetting: when someone was brought to Tuol Sleng their whole family was killed along with them, to eliminate the possibility of revenge.

The prison system (there were many of them across the country) was a machine that fed itself. Prisoners were forced to confess to being foreign spies, and then to name their collaborators. Under terrible duress, the prisoners would confess to anything, name everyone they knew as a traitor to the regime. The people they named were duly rounded up and named everyone they knew in turn. Some of the instruments of torture the Khmer Rouge employed are displayed at Tuol Sleng: a diagonal bed to which prisoners were shackled; a bath in which they were submerged. A medical program ran in tandem to the interrogations: people were drained of their blood for use in transfusions. There’s a shrine in which the skulls of some of the victims are displayed, the violence done to them still visible in the surface of the bone. There’s a steadily mounting sense of horror to the place. Here the ghosts are not those of absence but the ghosts of terror and violence.

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Sitting out in the courtyard was one of Tuol Sleng’s few survivors. Chum Mey was allowed to live because he possessed skills useful to his gaolers: he was good at fixing machines. Vietnam invaded Cambodia in late 1978, and drove the Khmer Rouge from Phnom Penh in January 1979; Chum Mey escaped in the resulting confusion. Now he sits in the place in which he was a prisoner, sharing his experience with visitors, making sure it’s not forgotten. My limited Khmer only allowed me to greet and thank him; even in English I’m not sure I could communicate how I admire his courage in returning to the prison – he’s there several times a week – and the warmth and wholeness that he radiates, the possibility of healing that he represents. It’s hardly possible to forget the Khmer Rouge era – between the executions and people starving in the fields they managed to kill two million people (or a quarter of Cambodia’s population at the time) – but I admire too the way the government acknowledges and memorialises the past at these prominent sites in the capital. The government’s relationship to that era is complicated; many in the ruling party (including the Prime Minister, Hun Sen) were members of the Khmer Rouge, and it has not always co-operated in bringing the worst perpetrators to trial. Any attempt to address the Khmer Rouge era can only be limited and symbolic, however, given the sheer number of people they killed. Tuol Sleng is a very potent symbol.

Later we took a tuk-tuk out to the Killing Fields. This was the execution ground for the prisoners at Tuol Sleng once they had confessed and informed on their associates. The work of imagination and empathy is harder there; local people stripped the buildings for their materials after the fall of the Khmer Rouge, and the mass graves, emptied of their occupants, are now gentle hollows in the ground. Except for the stupa memorialising the victims at its centre, it looks like a pockmarked park – paths lined by trees, a pond surrounded by bamboo. An excellent audio tour guided us around the site, explaining the significance of each area, finding the details to make it vivid. The Khmer Rouge executioners played patriotic music at volume to mask the sounds of the killing: at one point one such song played over my headphones, along with the grinding sound of the generators that powered the lights. These were the last sounds that the prisoners heard.

I’ve read criticisms of “dark tourism“: that there’s something distasteful about tourists trooping through concentration camps and execution grounds. Travel is (or should be) an exercise in empathy, in reaching out with thought and feeling to unfamiliar places and people. I think a sense of the Khmer Rouge era is basic to any understanding of Cambodia. In a larger sense too, the twentieth century was the story of experiments with totalitarian government. In Phnom Penh you can see where that leads.