An iron bar was buried next to the “toothless vampire skeleton” of Sozopol, as a sinister safeguard to keep it eternally sealed in its tomb and prevent it from re-emerging from the shadows.

In the late 18th century, the remains of eight human skeletons were found with stones placed in their mouths – something researchers believed locals had done to prevent the dead from returning to walk the Earth as zombies.

The investigation began more than six years ago at what was believed to be a medieval cemetery in County Roscommon, Ireland.

A few years ago, two skull malformations were discovered in Ireland with rocks placed in their mouths to prevent them from turning into zombies. One of the skulls, of an adult male in his twenties, was said to have been found among hundreds of other remains.

One of them was lying with his head facing upwards. A large black stone had been deliberately placed in his mouth.

‘The other had its head turned to one side and had an even larger rock jammed so violently into its mouth that its jaws nearly dislocated,’ he told Discovery News.

The two men were lying next to each other and were discovered together.

At first, the team thought they had stumbled upon a Black Death cemetery when the remains of people buried at the end of the Middle Ages had stones stuck in their mouths because they were involved in vampire hunting rituals. Around 3,000 skeletons buried from 700 to 1400 are thought to still be buried at the site.

The belief that zombies can return from the grave as the living dead has its roots in Haitian culture, where it is linked to voodoo magic and witchcraft.

Superstition is so strong on the Caribbean island that relatives of the deceased will dismember a corpse so that it cannot return as a zombie. They will even keep watch over the grave to protect the body until it decomposes.

According to believers, a zombie will rise from its grave in a hypnotic trance, able to respond to stimuli but lacking self-awareness.

Initially, archaeologists believed that the ritual of placing a stone in the mouth could be related to vampire hunting, where a stake is driven into the victim’s heart. Vampires were believed to spread plague and a stone placed inside the mouth was thought to prevent this. Practitioners also believed that placing a stone in the mouth of a corpse would starve it to death.

In 2009, the remains of a 60-year-old woman were discovered with a rock stuck in her mouth in a 16th-century mass grave on the Venetian island of Lazaretto Nuovo.

But vampire culture did not develop until the 16th century, and so does not explain why stones were found in 8th-century skeletons.

“In this case, the stones in the mouth may have acted as a barrier to prevent ‘revenants’ from returning from their graves,” Dr Read said.

The skeleton of a “vampire” buried in the 16th century has been revealed as part of a museum display.

Two years ago, a collection of bones was discovered in northern Poland bearing the telltale marks of a vampire burial ritual. Experts determined they were the remains of a man after they were found in an ancient cemetery in the town of Kamień Pomorski.

Now, they have been revealed as the main attraction of the Kamień Land History Museum, as organisers are preparing an exhibition exclusively for the “vampire”. The body was found with a stake driven into its leg and a small stone in its mouth.

Vampire “experts” claim that the stone was placed in the body’s mouth to prevent it from biting or sucking blood from its victims. And the piercing of the tibia and femur (usually with iron spikes) was done to prevent the vampire from leaving its grave.