It's been 60 years since a viking fortress was discovered, leading historians to assume that we'd uncovered everything there is to uncover. But this weekend, Danish and English archaeologists announced they've unearthed evidence of a new fortress that's been sought after for years—and they used some pretty cool tech to do it.

The fortress sits about 40 miles south of Copenhagen, at a point where researchers had believed a fort might lie. But until archaeologists turned to new methods offered by archaeological geophysics, they found nothing. Thanks to help from University of York researcher named Helen Goodchild, the team was able to discover evidence of a huge, circular fortress with four openings, each enclosed by wooden gates that were eventually burned down—presumably during battle. You can see the archaeologists posing with the remains of the posts here:

So how did the team discover the site? According to Aarhus University, Goodchild used a technique called gradiometry, which involves taking measurements of the Earth's magnetic field found in the soil at the site. By comparing variations from location to location, they were able to detect where humans had altered the Earth—and begin excavation, as professor Søren Sindbæk explained:

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By measuring small variation in the earth's magnetism we can identify old pits or features without destroying anything. In this way we achieved an amazingly detailed 'ghost image' of the fortress in a few days. Then we knew exactly where we had to put in excavation trenches to get as much information as possible about the mysterious fortress.

Here's what the imaging technique turned up, helping the team figure out exactly where to begin actually altering the land:

The ring itself was almost 500 feet wide, and inside, longhouses would've served as dwellings. It would've looked a lot like Trelleborg, another ringed fortress from the same period that sits 40 miles east of the new find:

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Image: Jørgen Falck

The researchers are still waiting to get lab results back on an analysis of the burned gate posts for an accurate dating of the fortress, but they speculate that it may date back to King Harald Bluetooth—or the son who deposed him, Sweyn Forkbeard, who ruled at the turn of the first millennia. Forkbeard would go on to become the King of England after invading London in 1013.

According to The Telegraph, this fortress may have been the one used to actually launch Forkbeard's final attack on England. We'll know more soon, and have reached out to the team for more information on the tools used in the discovery. [Aarhus University]