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Built on present-day Salisbury Plain in Wiltshire, England, between 3,000 and 1,500 B.C., the haunting stone circle known as Stonehenge has retained its air of importance and mystery for some 5,000 years. Over the centuries, people have speculated as to its purpose, proposing everything from prehistoric solar clock to ancient healing center to ceremonial burial ground. Many believe it to be a temple of some kind, but no one can say for sure who built it and whom it might be dedicated to.

A team of scientists added another layer to the enduring mystery surrounding Stonehenge. Using ground-penetrating radar and GPS-guided magnetometers to sweep the vicinity, the researchers of the Stonehenge Hidden Landscape Project were able to map the area for as far as two miles underground. The findings of the digital mapping project revealed a subterranean complex encompassing hundreds of features, including at least 17 monuments, from barrows and ditches to henges (circles of uprights stones or wood) and pits.

Among the most significant new discoveries relate to Durrington Walls, a bank of dirt stretching around a mile-long circumference located a short distance from Stonehenge itself. Though the area had been excavated in the past, the underground imaging of the area underneath one section revealed more than 60 large holes, now buried, in which stones would have sat. A number of the stones themselves also remained, each around 10 feet long. Dating to around the same period when Stonehenge was built, the stones and holes appear to form a “super henge” measuring some 1.5 kilometers around. As Professor Vincent Gaffney, an archaeologist at the University of Birmingham who co-led the project, told Nature: “That’s a big prehistoric monument which we never knew anything about.”

More surprising finds awaited the researchers in the Cursus, the rectangular ditch north of Stonehenge that is thought to have acted as a barrier for the stone circle. Measuring some 300 feet wide and just under two miles long, the Cursus was built several hundred years before Stonehenge. It got its name in the 18th century, when antiquarian William Stukeley observed that it resembled an ancient Roman racecourse. The Hidden Landscape Project’s work revealed gaps in the ditch, including a significant break in the northern side that would have allowed people to enter and exit. This find suggests that in addition to having an east-west orientation, along the path of the sun, the Cursus might also have guided peoples’ movement north to south.

In addition, the underground map revealed two large pits in the Cursus that appear to be aligned with two distinctive features of Stonehenge: the “Avenue,” a path that lines up with the rising sun on the summer solstice, and the “Heel Stone,” located at the entrance to Stonehenge, which aligns with the summer solstice sunset. According to Gaffney, these pits link Stonehenge with the Cursus directly for the first time. As he told LiveScience: “Suddenly, you’ve got a link between this very large monument and Stonehenge through two massive pits, which appear to be aligned on the sunrise and sunset on the mid-summer solstice.”

Taken together, the underground findings suggest that Stonehenge may be only the most obvious, aboveground remainder of a large complex of structures built with some ritual importance. No one has begun digging underground to verify the findings yet, but Gaffney has no doubt that his team’s work will influence any future excavations. “The important point is Stonehenge is not alone. There was lots of other associated ritual activity going on around it.”

READ MORE: Why Was Stonehenge Built?

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