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The megaliths of Stonehenge, which were raised above England's Salisbury Plain some 5,000 years ago, may be among the most extensively studied archaeological features in the world. Still, the monument is keeping secrets. Scientists just unveiled the results of a four-year survey of the landscape around Stonehenge that detected signs of at least 17 previously unknown Neolithic shrines.

"The important point is Stonehenge is not alone," project leader Vincent Gaffney, an archaeologist at the University of Birmingham in the U.K., told LiveScience. "There was lots of other associated ritual activity going on around it." [See Images of Hidden Stonehenge Monuments]

The images obtained through ground sensing techniques revealed hengiforms, or earthworks that are typically circular and surrounded by a ditch. Mario Wallner / LBI ArchPro

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Gaffney and his team revealed underground impressions, presumably left by wooden post holes, stones and ditches — some of which extend up to 13 feet (4 meters) deep. Images created with geophysical prospecting tools show that some of these smaller monuments had a concentric circle design, much like Stonehenge.

The researchers also peered inside the Cursus, an immense enclosure north of Stonehenge that dates back to about 3500 B.C. Stretching about 1.8 miles (3 kilometers) long and 330 feet (100 meters) wide, the Cursus had been deemed a barrier to Stonehenge, said Gaffney.

Red circles mark the spots where new monuments were discovered around Stonehenge. Researchers believe these archaeological features might have been satellite shrines involved in the processional and ritual activities at Stonehenge. Wolfgang Neubauer / LBI ArchPro

Researchers found a large pit buried on the eastern end of the Cursus that aligned with Stonehenge's "avenue," a processional path that lines up with the sun at dawn during the mid-summer solstice. The team found a matching pit at the other end of the Cursus that aligned with the Heel Stone at the entrance to Stonehenge, which is aligned with sunset during the solstice, Gaffney said.

The findings will be featured in a BBC Two series titled "Operation Stonehenge: What Lies Beneath," starting Thursday. A U.S. version of the special, "Stonehenge Empire," will air on the Smithsonian Channel on Sept. 21. The Stonehenge Hidden Landscapes Project is led by the University of Birmingham with the Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology.

— Megan Gannon, LiveScience

This is a condensed version of a report from LiveScience. Read the full report. Follow Megan Gannon on Twitter and Google+. Follow LiveScience on Twitter, Facebook and Google+.