Cooking was evidently a key skill for postwar China’s political dissenters – a good proportion of the interviewees in Wang Bing’s monumental oral history of Jiabiangou “re-education” camp survived only because of the extra scraps they scrounged working in the kitchens.

They had been sent to the Chinese equivalent of Siberia as part of the 1957 Anti-Rightist Campaign tabled by Mao Zedong to purge the country of Kuomintang influences, but which soon ended up sweeping up property owners, Christians, anyone who enjoyed exercising their critical faculties and, in one case here, a boy who drew tears on a portrait of the great leader. Between 500,000 and 1.3 million were caught in this ideological dragnet, 3,000 of whom found themselves doing hard labour and starving on the blasted plains at Jiabiangou, in Gansu province. Just 500 came back.

Filmed between 2005 and 2017, expanding on territory previously covered by Wang in Fengming (2007) and The Ditch (2010), Dead Souls compiles a gruelling eight-hour catalogue of human suffering. Inhabiting troglodytic shelters tunnelled out of the landscape, living on a handful of food a day, swollen and then shrivelled by malnutrition, the casualty rate was frightening. In winter, one of them recalls, death could come as quickly as a “blown-out lamp”.

If Wang’s spartan format doesn’t quite permit the panoramic historic relief of Shoah, the claustrophobic intensity of the interviews, circling around the same-but-unique brutality experienced by each inmate, gets across the vastness and indiscrimination of Mao’s dehumanisation machine. Strangely, these memories are often offered up with ironic smiles.

The futility of all this wasted human potential is what remains; something obvious in the bone-strewn ruins the survivors wander around in one section. Wang’s film is a vital excavation of history in danger of being eroded away.