For years, archaeologists were stumped. What happened to the Great Viking Army, a massive force that seized great swaths of England in the 9th century but left barely a trace?

Archaeologists now report that a mass grave in England may contain nearly 300 Viking warriors—the only remains of the Great Viking Army’s warriors ever found.

Archaeologists first uncovered the burial site in the 1980s, in Derbyshire, England, and thought it might contain remains from the Great Viking Army, also known the Great Heathen Army. But there was one problem—radiocarbon dating of the site revealed that the remains were too old to be Viking invaders.

The army was thought to have spent winters in Derbyshire from around 873 A.D. to 874 A.D, but initial analysis of the skeletons resulted in dates from the 7th and 8th centuries.

Now, a new study published in the journal Antiquity suggests those dates were incorrect, and that the timing is right for the remains to hail from the Great Army.

It’s clear that the Vikings had an impact in England. One sign of the Vikings’ Scandinavian influence is the presence of English towns ending in “-by,” derived from the Scandinavian word for village. The army's presence is also documented in the Anglo-Saxon Chronicle , says lead study author Catrine Jarman from the University of Bristol.

Yet so far, the gravesite at Derbyshire is the only burial site found that has been linked to the army.

“That is one of the greater mysteries,” says Jarman. “We have thousands of thousands of people entering, but there's so little physical evidence.”

Fishy Bones

The new study clears up the date discrepancy by taking into account one crucial detail: The Vikings, famous for seafaring, had a high-seafood diet. This, Jarman notes, skews radiocarbon tests.

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“It's a phenomenon that we only started realizing,” she says.

When scientists date human bones, they look at the amount of radioactive carbon-14 present. This form of carbon decays over time, so the amount of carbon-14 in bones can tell scientists how long it’s been since the bones formed. However, people who eat large amounts of seafood are subject to what Jarman calls “marine reservoir effects.”

“If you eat fish, then some of the carbon has come from the ocean. Some of these Vikings were eating a lot of fish, so that affects carbon dating,” she says.

For comparison, Jarman notes that if a Viking had killed a fish and a sheep on the same day, radiocarbon dating would make it appear as if the fish had died 400 years before the sheep.

To determine how much the Vikings’ diet may have offset the initial carbon dating, the researchers performed an initial chemical analysis on 17 individuals from various locations at the massive gravesite, as well as a sheep's jaw found at the gravesite.

Jarman now feels comfortable saying that nearly all the bones date to the late 9th century, making it a strong possibility that the bones come from the Great Viking Army.

The Warriors’ Remains

“It was what the archaeology seemed to suggest all along,” says Jarman.

Between 1980 and 1986, a series of excavations uncovered a burial mound called a charnel that contained the remains of 264 people. Of those, more than 80 percent of the bones are male, and many show signs of violent injury.

Among the remains, archaeologists also found evidence of warfare such as axes and knives. (Not all Viking warriors were men. DNA recently confirmed one famous warrior was a woman.)

A separate double grave nearby was also found with the remains of two men who were buried with a pendant of Thor's hammer and a Viking sword.

In addition to the gravesites, archaeologists also found evidence of what may have been a large defensive ditch.

With the new carbon dates, Jarman says archaeologists can't say with 100 percent certainty that the gravesite belonged to the Great Viking Army, but evidence strongly suggests it.