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 the University of Birmingham in the U.K. "It will completely change the way we think about the landscape around Stonehenge."
Using the latest geophysical imaging technology, Gaffney's team captured digital impressions of the now buried remains of the newfound henge formation, just over half a mile (900 meters) from its world-famous neighbor.
Measuring 82 feet (25 meters) wide, the circular feature had a segmented ditch dotted with 20 or so large holes—suspected to have been postholes for a timber, rather than stone, circle, the team says. (Also see "Wooden 'Stonehenge' Emerges From Prehistoric Ohio.")
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," Gaffney told National Geographic News (interactive Stonehenge time line).
Currently the leading view is that the immediate area around Stonehenge was a sacred, off-limits area where only a select few, such as high priests or nobility, were allowed. (See "Stonehenge 'Hedge' Found, Shielded Secret Rituals?")
"If [the newfound henge] was there at the same time, it demonstrates there was massively more activity going on in the landscape adjacent to Stonehenge," Gaffney said.
That isn't to say Stonehenge was open to anybody, he added, "but we are saying there seems to be more activity inside that boundary.
"Stonehenge," he added, "is one of the most studied monuments on Earth but this demonstrates that there is still much more to be found."
Stonehenge and Sister Circle Linked to Solstice
The team suggests the supposed wooden henge, like Stonehenge, performed an important ceremonial role for ancient Britons who gathered at the summer and winter solstices to mark the passing year with sacred rituals.
Also like Stonehenge, the now vanished henge is oriented toward 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, as archaeologist with English Heritage, the government agency that manages the Stonehenge World Heritage site.
More Stonehenge Circles to Be Found?
The suspected new henge—or ring-shaped mound with an adjacent ditch—adds to a growing number of stone and timber circles found in the area.
One of the best known Stonehenge sisters, Woodhenge, was discovered in 1925 and lies about 2 miles (3.2 kilometers) from Stonehenge. In 2009 the remains of a 33-foot-wide (10-meter-wide) circle of 25 stones, dubbed Bluestonehenge, was uncovered just over a mile (1.6 kilometers) from Stonehenge.
An English Heritage geophysical survey last year uncovered another Stonehenge-area circle, though it hasn't yet been revealed to the public, according to Joshua Pollard, an archaeologist at the University of Bristol.
New Circle May Have Been a Stone Henge—Not Timber
The age and full significance of the latest Stonehenge discovery won't be known until actual digging takes place, according to Pollard, who isn't involved in the new Stonehenge survey.
"The less exciting interpretation is that it's just a peculiar Bronze Age barrow"—a type of burial mound—he said.
Or, if the monument does prove to be an ancient temple, it could be that, like Bluestonehenge, the newfound site held a long-gone stone circle, not a timber one, he added.
Pollard said scattered fragments of bluestone have been found in the same area as the new henge. Prehistoric builders transported bluestone—a collective name for stone not native to the Stonehenge region—more than 155 miles (250 kilometers) from southwestern Wales to the Stonehenge area to build the massive stone circles.
"People have suspected there may have been a stone circle there which no longer survives," he said. "There's an intriguing possibility that what they might have picked up is actually the site of another stone circle."
Survey leader Gaffney doesn't rule out the idea that Stonehenge's smaller twin was also made of stone. But he said he would expect the pit holes and surrounding ground to look more disturbed if they had held stones that were later dragged out or broken up.
"To me they look like timbers which have rotted in situ," he said.
(See Stonehenge pictures.)
Swifter Surveys at Stonehenge
For this first ever systematic geophysical survey of the area surrounding Stonehenge, the survey team employed a new, faster method of surveying beneath the ground using a combination of radar imaging and magnetometry, a technique that maps changing patterns of magnetism in the soil.
The new henge was found in just the first two weeks of a three-year project to map 5.5 square miles (14 square kilometers) of the Stonehenge landscape.
(Take a Stonehenge quiz.)
Assuming the find wasn't a fluke, the team's swift success hints that such circles may have been fairly common around Stonehenge.
"If we do have a number of small timber and stone circles in that landscape—which on current evidence is looking quite likely—then I suspect the survey will pick them up," the University of Bristol's Pollard said.
"We could be looking at a very different perception of the kinds of monuments within the Stonehenge area than we have at present," he added.
In fact, survey leader Gaffney said he expects other monuments to turn up, sites that, like the newfound henge but unlike other known circles, are visible from Stonehenge.
Archaeologist Andrew Fitzpatrick, who wasn't part of the survey, said such prehistoric sites are easily missed, having been "ploughed flat over many thousands of years."
Given the survey team's almost immediate success, added Fitzpatrick, of the nonprofit consulting group Wessex Archaeology, "we should expect many more exciting discoveries as the project continues."