The Neolithic circle of giant stones has been a source of wonder and mystery for nearly 5,000 years — in the Middle Ages, the wizard Merlin of Arthurian legend was said to have stolen the monument from Ireland.

More recently, scientists have determined that the site’s upright sandstones came from relatively nearby Marlborough, while the bluestones arrayed near its centre came from Wales.

But the origin of the Altar Stone, a unique six-tonne slab laying on its side at the heart of the circle, remained elusive.

It was long thought to have also come from Wales, but tests along those lines always “drew a blank,” said Richard Bevins, a professor from Aberystwyth University, mid-Wales, and co-author of a new study.

This prompted a team of British and Australian researchers to broaden their horizons — and in turn discover something “quite sensational”, he told AFP.

Using chemical analysis, they determined that the Altar Stone came from Scotland’s Orcadian Basin, which is at least 750 kilometres (460 miles) from Stonehenge, according to the study in the journal Nature.

Genuinely shocking

The researchers were stunned.

“This is a genuinely shocking result,” study co-author Robert Ixer of University College London said in a statement.

The “astonishing” distance was the longest recorded journey for any stone at the time, said fellow co-author Nick Pearce of Aberystwyth University.

Whether people around 2,500 BC were capable of transporting such huge stones from Wales had already been a matter of heated debate among archaeologists and historians.

That a five-by-one-metre (16-by-three-feet) stone made the trip across much of the length of the UK suggests that the British isles were home to a highly organised and well-connected society at the time, the researchers said.

They called for further research to find out exactly where in Scotland the stone came from — and how it made its way to Stonehenge.

One theory is that the stone was brought to southern England not by humans but by naturally moving ice flows.

However research has shown that ice would actually have carried such stones “northwards, away from Stonehenge”, lead study author Anthony Clarke from Australia’s Curtin University told a news conference.

Another option was that the Neolithic builders moved the stones over land — though this would have been extraordinarily difficult.

Dense forest, marshy bogs and mountains all formed “formidable barriers” for prehistoric movers, Clarke said.

Incredibly important

Another option is that the stone was transported by sea.

There is evidence of an “extensive network of Neolithic shipping,” which moved pottery and gems around the region, Clarke said.

To work out where it came from, the researchers fired laser beams into the crystals of a thin slice of the Altar Stone.

The ratio of uranium and lead in these crystals act as “miniature clocks” for rocks, providing their age, said study co-author Chris Kirkland of Curtin University.

The team then compared the stone’s age to other rocks across the UK and found “with a high degree of certainty” that it came from the Orcadian Basin, Kirkland said.

Susan Greaney, an archaeologist at the UK’s University of Exeter not involved in the study, said it established the first “direct link” between southern England and northern Scotland during this time.

“The placement of this stone at the heart of the monument, on the solstice axis, shows that they thought this stone, and by implication, the connection with the area to the north, was incredibly important,” she told AFP.