© Ludwig Boltzmann Gesellschaft (Vienna)
Scientists say they had no idea massive henge of 50 stones was just two miles away.
Archaeologists have discovered that Stonehenge had a huge stone sibling just two miles to the northeast.

Using powerful ground-penetrating radar, which can 'x-ray' archaeological sites to a depth of up to four metres, investigators from Birmingham and Bradford universities and from the Ludwig Boltzmann Institute in Vienna have discovered a 330 metre long line of more than 50 massive stones, buried under part of the bank of Britain's largest pre-historic henge.

"Up till now, we had absolutely no idea that the stones were there," said the co-director of the investigation Professor Vince Gaffney of Birmingham University.

© Ludwig Boltzmann Gesellschaft (Vienna)
Investigators discovered evidence of dozens of new monuments close to Stonehenge.
The geophysical evidence suggests that each buried stone is roughly three metres long and 1.5 metres wide and is positioned horizontally, not vertically, in its earthen matrix.

However, it's conceivable that they originally stood vertically in the ground like other standing stones in Britain. It is thought that they were probably brought to the site shortly before 2500BC.

They seem to have formed the southern arm of a c-shaped ritual 'enclosure', the rest of which was made up of an artificially scarped natural elevation in the ground.

The c-shaped enclosure - more than 330 metres wide and over 400 metres long - faced directly towards the River Avon. The monument was later converted from a c-shaped to a roughly circular enclosure, now known as Durrington Walls - Britain's largest pre-historic henge, roughly 12 times the size of Stonehenge itself.
Ground Penetrating Radar
© Ludwig Boltzmann Gesellschaft (Vienna)
The ground-penetrating radar can detect archaeological sites to a depth of four metres.
As a religious complex, it would almost certainly have had a deeply spiritual and ritual connection with the river. But precisely why is a complete mystery, although it is possible that that particular stretch of water was regarded as a deity.

The discovery of the buried stones is part of a much wider archaeological investigation into Stonehenge's sacred landscape.

A two-part special BBC Two documentary (Operation Stonehenge: What Lies Beneath), being shown this Thursday evening and next Thursday, is set to reveal the details of many of the investigation's new discoveries.

As well as revealing the previously unknown stones of Durrington Walls, the Anglo-Austrian-led investigation has succeeded in locating more than 60 other previously unknown pre-historic monuments.

"It shows that, in terms of temples and shrines, Stonehenge was far from being alone," said Professor Gaffney.

Using ground-penetrating radar, magnetometry and other geophysical techniques to peer beneath the landscape's surface, archaeologists have found around 17 other henge-like Neolithic and Bronze Age religious monuments, each between 10 and 30 metres in diameter. Some may well have consisted of circles of large timber posts - wooden equivalents of conventional prehistoric stone circles.

But the archaeologists have also discovered around 20 large and enigmatic ritual pits - each up to five metres in diameter.
Salisbury Plain
© Ludwig Boltzmann Gesellschaft (Vienna)
They have also discovered more than half a dozen previously unknown Bronze Age burial mounds - and four Iron Age shrines or tombs, as well as half a dozen Bronze Age and Iron Age domestic or livestock enclosures.

In total, some 4.5 square miles of buried landscape has been surveyed by the joint Birmingham/Vienna team in an exercise that has taken four years to complete.

Now the archaeologists plan to analyse the new data - in order to work out how all the newly discovered prehistoric monuments related to each other.

Using avatar-based computer models, they are hoping to tease out exactly how Neolithic and Bronze Age people used Stonehenge's landscape.

Initial results suggest that some of the newly discovered shrines and other monuments grew up along processional ways or pilgrimage routes in Stonehenge's sacred landscape.

The 4.5 square mile survey is the largest of its kind ever carried out anywhere in the world.

The large variety of 'x-ray' style techniques used have included more than half a dozen different systems.

Magnetometry and electro-magnetic induction have been used to map underground features by firing electro magnetic energy into the ground and then measuring the inter-action of that energy with subterranean features such as buried pits, ditches and stones.

Earth resistance and electrical resistivity imaging have gathered data on underground features by firing electrical energy into the ground and measuring differences in sub-surface resistance to it.

A fifth technique, magnetic susceptibility analysis, helps archaeologists detect buried layers of burnt material, which often indicate ancient human activity. The system works because naturally occurring iron oxides in the ground can become magnetized through the process of being burnt. A final technique, microgravimetry, can also help detect subterranean features, especially cavities - by measuring tiny differences in local gravitational fields.

The four year investigation into what lies beneath Stonehenge's landscape has been carried out jointly by four UK universities (Birmingham, Bradford, St. Andrews and Nottingham) and two continental European institutions - the University of Ghent in Belgium and the Ludwig Boltzmann Institute in Austria.