The largest Altar Stone at the heart of Stonehenge is actually Scottish not Welsh, research suggests.
The "remarkable" discovery has left experts "stunned" and busts a century-long held belief about the origins of the bluestone at the centre of the prehistoric monument.
It is unclear when the stone arrived at the site on Salisbury Plain in Wiltshire, but the scientists say it may have been placed within the central horseshoe of stones during the second construction phase at about 2620-2480 BC.
It is also not known whether the now horizontal stone once stood upright.
The new discovery implies that one of the most famous stones in the world was moved much further than had been believed - at least 435 miles.
For the last century, the six-tonne sandstone, at the heart of the ancient site, was believed to have come from Wales, as the majority of Stonehenge's bluestones came from the Preseli Hills area in west Wales and are believed to have been the first stones erected at the site.
The stone at the focus of this study has traditionally been grouped with these other, smaller bluestones.
However, the true origin of the Altar Stone had remained a mystery, until now.
After work to examine the stone's chemical composition and mineral grains, scientists can say with 95% confidence that the sandstone is very likely to have come from north-east Scotland.
The study involved scientists at Aberystwyth University, University College London and, in Australia, Curtin University and the University of Adelaide.
The findings indicate a striking similarity between the Old Red Sandstone of the Orcadian Basin in north-east Scotland and the Stonehenge Altar Stone.
Co-author Professor Richard Bevins, from Aberystwyth University, said: "These findings are truly remarkable - they overturn what had been thought for the past century.
"We have succeeded in working out, if you like, the age and chemical fingerprints of, perhaps, one of the most famous of stones in the world-renowned ancient monument.
"It's thrilling to know that our chemical analysis and dating work has finally unlocked this great mystery.
"We can now say that this iconic rock is Scottish and not Welsh.
"Although we can say that much, and confidently - the hunt will still very much be on to pin down where exactly in the north-east of Scotland the Altar Stone came from."
The scientists used their analysis of the ages of the mineral grains in the stone to essentially create a fingerprint of the source of those grains.
They matched ages found in rocks of the Orcadian Basin found in the north-east of Scotland, and are completely different from Welsh-sourced stones.
Anthony Clarke, from the Timescales of Mineral Systems Group at Curtin University, in Perth, Western Australia, said: "Considering the technological constraints of the Neolithic, our findings raise fascinating questions about how such a massive stone was able to be transported over the vast distance implied.
"Given major overland barriers en route from north-east Scotland to Salisbury Plain, marine transport is one feasible option."
Commenting on the findings, he added: "All four of us were stunned. We couldn't believe it."
Co-author Dr Robert Ixer, of the UCL Institute of Archaeology, said: "This is a genuinely shocking result, but if plate tectonics and atomic physics are correct, then the Altar Stone is Scottish.
"The work prompts two important questions: why and exactly how was the Altar Stone transported from the very north of Scotland, a distance of more than 700 kilometres, to Stonehenge?"
The research, published in the journal Nature, does not provide direct evidence about how the stone got to its world-famous location in Wiltshire.
But the revelation that it travelled so far will raise questions about its journey given the limits of human technology during Neolithic times.
The Altar Stone is classed as a bluestone, but is very different from the other ones at Stonehenge.
It is about twice the size of the smaller igneous bluestones, weighs six tonnes and is about five metres long, while the others are about a maximum of three tons.
Professor Nick Pearce, from Aberystwyth University, said: "This stone has travelled an awful long way - at least 700 kilometres - and this is the longest recorded journey for any stone used in a monument at that period. The distance travelled is astonishing for the time.
"While the purpose of our new, empirical research was not to answer the question of how it got there, there are obvious physical barriers to transporting by land, but a daunting journey if going by sea.
"There's no doubt that this Scottish source shows a high level of societal organisation in the British Isles during the period.
"These findings will have huge ramifications for understanding communities in Neolithic times, their levels of connectivity and their transport systems.
"Hopefully, people will now start to look at the Altar Stone in a slightly different context in terms of how and when it got to Stonehenge, and where it came from.
"I am sure this will lead to some new thinking about the development of Stonehenge and its links to the rest of Neolithic Britain."
