The stone in the mouth might be to make a supernatural barrier between the worlds of the dead and the living.

In summer 2007, at a site in Poland, archaeologists were uncovering layers of graves. They were astonished to find two skeletons with large stones lodged in their jaws.

This was exceptional; all the other skeletons in the cemetery had been normal Christian burials. The archaeologists believe that the skeletons were treated as if they were ‘revenants’ – individuals who might return from the dead to prey on the living.

The film attempts to discover who these people were, how they died, and when the stones were forced into their jaws. It’s a fascinating journey first into the medieval mind, and back to our earliest fears of the undead. And we come up to date, with an interview with a Romanian villager who saw his neighbours cutting the heart out of a corpse that they believed to be a vampire.

Fear of vampires was strong among Eastern Europeans in the medieval period.

Professor Martyn Rady, a colleague of Hjelm’s, posits the folk tales spread from Serbia to surrounding countries following a report sent by Austrian military authorities to their superiors in Vienna. It told of a mercenary soldier who had been turned into a vampire and infected his victims, who when exhumed from their own graves were found to have fresh blood in their cavities. The report was intercepted before reaching the Austrian capital and published in newspapers.

The Polish had particular reason to fear vampires rising from their graves, Hjelm explains.

“According to some sources, Poles thought that vampires were born, rather than ‘made’,” he says. “They were normal people who could live normal lives, not aristocrats living in distant castles. The problems only started when these people died. They could come back to live with their families and even impregnate their wives.”

However, some experts warn against presuming all such unconventional burials were because of a fear of vampires.

Vampiric or not, the strange range of historical graves in indicate the country held deep beliefs around the ritualistic burial of certain citizens. Leszek Gardela, from the Institute of Archaeology at the University of Rzeszów, is currently working on a research project into the phenomenon.

“Unusual mortuary practices have fascinated Polish scholars since the 1950s (when the first were discovered),” she says. “But so far they remain unknown to a wider international audience. It is high time to change this.”