The surprising and gruesome discovery of smashed human skulls left on spikes in Sweden 8,000 years ago has left a vivid impression on baffled Scandinavian archaeologists as they try to unravel the mystery of this group of individuals who lived and died during the Mesolithic Period.

The skulls were originally found sometime during the years 2009 to 2011 while a bridge was being constructed over the Motala Strom River and work was being done in Kanaljorden.

It was here in a lake in Kanaljorden that a group of archaeologists found relics which included tools crafted from the antlers of different animals, along with the stakes and 8,000-year-old human skulls.

Up until this point in time, it was believed that those who lived during the Mesolithic Period would have had the utmost respect for the dead bodies of humans, yet the discovery of these smashed skulls in Sweden showed that this may not have been the case for this particular group of people, which thoroughly confused the archaeologists from the Cultural Heritage Foundation who first found the remains.

As archaeologist Fredrik Hallgren explained, his team expected to find the usual items like animal bones, but never would have guessed they would come across anything like they did, as National Geographic reports.

“We had hopes of finding animal bones, but not this rich complex.”

The remains belonged to Mesolithic hunter-gatherers known to show respect for their dead—until now.https://t.co/zdLzFguYYP — National Geographic (@NatGeoMag) February 14, 2018

Nine smashed skulls of adults were found in Kanaljorden, but the skull belonging to an infant was also discovered with those of the adults. No jaw bones remained on these skulls, and two of them still had spikes driven through them. These stakes had been placed into the bottom of their skulls, which archaeologists believe occurred before they were placed in the lake.

Interestingly, animal bones were found to be positioned around this group of human skulls, and each animal species was kept together, as Hallgren noted.

“They somehow seem to differentiate between humans and animals but also animals in different categories.”

Archaeologists have determined that four of these skulls belonged to males while two were female, and also found that two of the Mesolithic era humans would have been approximately 20 to 35-years-old at the time of their deaths. The infant child, however, is believed to have died immediately after its birth, although there is also the distinct possibility that it could have been stillborn.

One of the things about these smashed skulls that most confused archaeologists is that while all of the humans clearly suffered trauma to the head, over half of them appeared to have healed from their injuries before death, as Fredrik Hallgren explained.

“These are not people who have been recently smashed in the head and then put on display. More than half of them had this healed trauma to the head.”

The exact reason for the blunt force injuries to these individuals remains unclear, but it may have occurred during times of village raids or other acts of warfare, according to Science Alert. Archaeologists have also concluded that there is a strong possibility that these human remains were first buried elsewhere and then later on removed to be placed in the lake with the wooden stakes, 400 of which have been recovered from the site.

But whatever the reason for placing these 8,000-year-old skulls on spikes in the lake, it was clearly done with a specific purpose in mind, as archaeologists relate in their research.

“The deposition can be described as being carefully planned and executed, from the construction of the underwater stone packing to the spatially separated depositions of curated human and animal remains. The fact that the majority of the individuals show healed injuries seems to be more than a coincidence and implies that they were specifically chosen for inclusion in the deposition.”

Archaeologists continue to work on this case to solve the mystery of the 8,000-year-old skulls found on spikes in Sweden, but for now, you can read their latest research in the Cambridge University journal Antiquity.