The discovery, made completely without digging, suggests that now solitary Stonehenge may have been surrounded by "satellite Stonehenges," archaeologists say.
"This finding is remarkable," said survey-team leader Vince Gaffney, an archaeologist at the University of Birmingham in the U.K. "It will completely change the way we think about the landscape around Stonehenge."
—With reporting by James Owen in London
Illustration courtesy University of Birmingham
Stonehenge: Solstice Central
Each summer solstice the sun rises above Stonehenge's outlying Heel Stone (at top center), casting its rays into the center of the monument. The newfound henge is also oriented to the solstice sunrise, with entrances to the northeast and southwest.
"This new monument is part of a growing body of evidence which shows how important the summer and winter solstices were to the ancient peoples who built Stonehenge," said Amanda Chadburn, an archaeologist with English Heritage, the government agency that manages the Stonehenge World Heritage site.
Stonehenge's newfound sister henge—marked as the "New Henge" in the map above—may add to the slowly emerging picture of the region's prehistoric spiritual landscape.
Stonehenge itself is linked to a timber circle called Woodhenge via the River Avon and an ancient road called the Avenue. Though close to Stonehenge, the newfound site is even closer to the Cursus, a huge, stretching earthwork dated to around 3,500 B.C.
"Before Stonehenge was constructed, [the Cursus] was the principal monument in that landscape," said Joshua Pollard, an archaeologist at the University of Bristol who isn't involved in the new survey.
It's possible that the Stonehenge complex started off "with a series of small stone circles which are actually focused on the Cursus itself," Pollard said.
For this first ever systematic geophysical survey of the area surrounding Stonehenge, the survey team pulled ground-penetrating instruments behind an ATV in July, as pictured.
The new henge was found via a combination of radar imaging and magnetometry, a technique that maps changing patterns of magnetism in the soil.
The method was surprisingly successful, and surprisingly fast. The new site emerged within the first two weeks of the three-year project to map 5.5 square miles (14 square kilometers) of the Stonehenge landscape.
The circle's estimated date of 2,500 to 2,200 B.C. suggests "it was operating when Stonehenge was in its final and most dramatic form," survey leader Vince Gaffney told National Geographic News (interactive Stonehenge time line).
Magnetometry reading of underground pits near Stonehenge. Image courtesy University of Birmingham.