LONDON (AFP) - A vast complex of monuments surrounding Britain's prehistoric Stonehenge site has been revealed using high-tech underground scanning, archaeologists said on Wednesday.

The mysterious circle of standing stones, on Salisbury Plain in south-west England, is one of the most iconic ancient sites in Europe and was long thought to stand alone. But high-resolution scanning of the 12 sq km around it, penetrating 3m below the ground, has found it was surrounded by 17 neighbouring shrines.

"Stonehenge is the most iconic archaeological monument, possibly along with the pyramids, on the planet," project leader, Professor Vincent Gaffney, told the British Science Festival in Birmingham, central England. "Most of the area around Stonehenge is terra incognita. It has never been explored and everything we think about Stonehenge is on the basis of what we don't know about it.

"This is going to change how we view Stonehenge. It is not yet another find from Stonehenge, it's a fundamental step forward in the way we understand it."

The four-year study used magnetometers - advanced metal detectors - ground-penetrating radar, electromagnetic sensors and three-dimensional laser scanners.

It uncovered finds dating back 6,000 years, including evidence of 17 previously unknown wooden or stone structures as well as dozens of burial mounds.

They include giant pits, some of which appear to form astronomical alignments.

The nearby Durrington Walls "super-henge", which has a circumference of nearly 1.5km, was once flanked by up to 60 posts or stones up to 3m high, the scans showed.

Many burial mounds were found, including one barrow 33m long, within which signs of a giant timber building were found.

It is suggested this was the site of complex rituals involving the dead, including the removal of flesh and limbs.

"This project has revealed that the area around Stonehenge is teeming with previously unseen archaeology and that the application of new technology can transform how archaeologists and the wider public understand one of the best-studied landscapes on Earth," said Prof Gaffney.

"New monuments have been revealed, as well as new types of monument that have previously never been seen by archaeologists."

A Unesco world heritage site, Stonehenge is one of the most impressive prehistoric megalithic monuments anywhere due to its size, sophisticated concentric plan and architectural precision.

It is suggested that the layout formed a theatrical arrangement, with Stonehenge gradually emerging from the landscape as visitors walked towards it on an ancient processional route.