Five thousand years after the first monument was created at Stonehenge, it continues to give up dramatic new secrets - such as the "jaw-dropping" revelation earlier this year that its central stone had been transported more than 700km to Salisbury plain from the very north of Scotland.
While it had been known for more than a century that the huge sarsens for which Stonehenge is best known for come from more than 12 miles (20km) away and its "bluestones" originated in Wales, the discovery that the altar stone, which sits right at its heart, was Scottish caused an archaeological sensation, capturing headlines around the world.
Now, a leading archaeologist and authority on Stonehenge has proposed a striking explanation of why its stones were transported such mind-boggling distances.
Stonehenge may have been erected explicitly to unite early farming communities across the island of Britain at a time of cultural stress, argues Mike Parker Pearson, professor of British later prehistory at University College London - and the altar stone may have been taken from a distant Scottish monument as a gift or marker of political alliance.
While the structure is unique for many reasons - including its extensive stone-dressing and that it has lintels - no other monument in Britain or Ireland incorporates stones that were brought such huge distances, Parker Pearson writes in a new paper. "Stonehenge stands out in being a material and monumental microcosm of the entirety of the British Isles."
As such, we should consider Stonehenge to be a political monument as much as a religious one, Parker Pearson said. "It's not a temple - that has been a major stumbling block for hundreds of years. It's not a calendar, and it's not an observatory." The structure's famous alignment to the winter and summer solstices echoes earlier constructions such as Newgrange in the Boyne valley in Ireland, but may not be its main purpose, he said.
"I think we've just not been looking at Stonehenge in the right way. You really have to look at all of it to work out what they're doing. They're constructing a monument that is expressing the permanence of particular aspects in their world."
The altar stone is often overlooked by visitors to Stonehenge because it lies flat and partly obscured by a huge fallen sarsen. It was long assumed that it had also fallen, said Parker Pearson - yet north-east Scotland is home to multiple circles in which the stones are purposely laid flat.
"Given what we now know about where it's from, it seems all the more likely that it was deliberately set as a recumbent stone," he said, adding that it is "highly likely" that the altar stone had been part of an earlier Scottish monument. "These stones are not just plucked out of anywhere."
With Orkney having been ruled out as a potential origin, other potential sites or origin are now being examined. "I think we'll wait and see. It's very exciting," he said.
Archaeologists believe the altar stone may have been installed around 2500BCE, about the time Stonehenge was being remodelled from its original form.
This was a period of cultural change in Britain amid new arrivals from mainland Europe. "There's obviously some kind of interaction - you might call it first contact," said Parker Pearson. "That is the moment that Stonehenge is built, and I wonder if it is that moment of contact that serves, in whatever way, as the catalyst for this really impressive second stage of Stonehenge. It's an attempt to assert unity, quite possibly integrating the newcomers - or not."
Ultimately, however, "it doesn't succeed" - given genetic research shows the incoming "beaker people" would largely displace the earlier neolithic populations. "That said, Stonehenge is adopted [as a monument] by those beaker-using people whose descendants become the dominant population of Britain," said Parker Pearson.
"So despite the change in population, Stonehenge continues to exert its significance in the wider world."