A huge ring of 4,500-year-old pits discovered about two miles from Stonehenge were made by humans, archaeologists have confirmed. Twenty times bigger than Stonehenge—making it the largest prehistoric site in the U.K.—the circle of underground structures measures 1.2 miles in diameter, with the Durrington Walls, a Neolithic enclosure that once contained another henge, at their center.

Scientists used new optically stimulated luminescence (OSL) to identify 20 shafts so far, which all measure 10 meters wide and 5 meters deep. The fact that they all have the same dimensions supports the idea that they were man-made.

“The remarkable consistency across the cores, the identification of multiple and distinct fills, the suggestion that the pits were infilled at a similar time,” showed these were not natural features, Dr Tim Kinnaird of the school of earth and environmental sciences at the University of St Andrews in Fife, Scotland, who conducted the tests, told The Guardian. “It’s confirmed that the [pits] are all very similar, which is fascinating.”

A Google Earth image edited show the 20 late Neolithic period pits discovered encircling Durrington Walls. Image courtesy of EDINA Digimap Ordnance Survey Service.

The proof vindicates Prof Vincent Gaffney, of Bradford University, who led the team that first discovered the pits last year, and believed they were evidence of a massive prehistoric site nicknamed “Superhenge”. Some experts at the time, however, dismissed the shafts as natural sinkholes.

“We’ve now looked at nearly half of them and they’re all the same,” he told The Guardian. “So effectively this really does say this is one enormous structure. It may have evolved from a natural feature, but we haven’t located that. So, it’s the largest prehistoric structure found in Britain.”

The news could further disrupt plans to build a tunnel through the area, to improve local traffic issues. Campaigners scored a win earlier this year, when development of the tunnel was put on hold, but this latest discovery could mean there are even more prehistoric secrets hidden below the ground around Stonehenge.