LONDON — The world’s most famous prehistoric monument continues to surprise and amaze. Researchers on Wednesday revealed that the long-mysterious “Altar Stone” at the heart of Stonehenge came from faraway Scotland, raising tantalizing new questions about how – and why – a six-ton slab of sandstone made its way from north to south 5,000 years ago.

Aliens? Not likely, unless the government knows something we don’t. Could it have been deposited nearby by ice-age glaciers that covered Britain during the woolly mammoth Pleistocene? Probably not. The ice sheets in Scotland were moving north, not south, and so deposited their moraines in the opposite direction.

So we are left with two theories.

The world heritage site of Stonehenge is seen in Wiltshire, England in 2013. Alastair Grant/Associated Press, file

The Altar Stone was dragged 500 miles or more overland by our neolithic forebears, before the invention of the wheel, across the high hills and through the dense forests of prehistoric Britain, or, as these scientists speculate, the stone was ferried via sea by Stone Age mariners, a routing that “demonstrates a high level of societal organization with intra-Britain transport during the Neolithic period.”

Meaning? That neolithic peoples were capable of delivering a Stone Age prehistoric FedEx package weighing 13,227 pounds from as far north as the Orkney Islands off the coast of Scotland to the Salisbury Plain southwest of present-day London, with the understanding that six tons is approximately the weight of a full-grown male African elephant.

Such a delivery might have taken a decade or more – but “just in time” is relative.

At this point, the scientists say your guess is about as good as theirs, which is why Stonehenge remains “a wonder of the world.”

The new research, led by scientists at Curtin University in Australia, is jaw-dropping – upending a century of geological inquiry to conclude that the Altar Stone, long believed to originate from Wales, actually hails from Scotland, and would have had to make a trip of 450 to 600 miles.

The stone must have meant a lot to the people back then. There is other evidence of other stones being moved from stone circle to stone circle.

“But this is a very, very long way,” said Heather Sebire, senior curator at Stonehenge for the group English Heritage. She called the new findings “amazing.”

What was Stonehenge – a burial site, a meeting place, an astronomical clock or “computer?”

The Altar Stone has always been a spooky outlier, unique in its composition and its placement, and one of the most analyzed pieces of stone ever, a kind of moon rock for the archaeologist set.

You can visit it today. It lies at the center, the heart, of the stone circle but was found recumbent, lying flat, on the horizontal. When did it arrive? Maybe 2500 B.C. Could it have come to Stonehenge later? Sure, but the builders might have had to squeeze it into place.

Nobody knows whether the Altar Stone served as an altar, just as the nearby “Slaughter Stone” probably had nothing to do with sacrifice – human or otherwise. These was just the romantic (lurid) names that the early pearl-clutching antiquarians gave the rocks, imagining druids doing their nasty business to appease some unholy deities.

Stonehenge’s Altar Stone lies underneath two Sarsen stones in Wiltshire, England earlier this month. Nick Pearce/Aberystwyth University via AP

A 25-year-old PhD student named Anthony Clarke, who was originally from Wales and visited Stonehenge as a child, led the search for the provenance of the Altar Stone.

Clarke and colleagues examined two polished wafer-thin samples of the stone, thinner than a strand of hair – and wow.

Robert Ixer, an archaeologist at University College London, an authority on the geology of Stonehenge and a co-author of the paper that appears in the journal Nature, said the team in Australia had just two samples of the Altar Stone to work with.

“But that was enough,” he told The Washington Post.

He called the new provenance of the Altar Stone “astonishing.” Ixer spent much of his career assuming the Altar Stone was from Wales, still a far distance, but not nearly as far away as Scotland.

So precious is the neolithic monument, a World Heritage site, that it is no longer possible to take a hammer and chisel, as the Victorians and their predecessors did, and knock off a chunk to examine.

Ixer explained that the two samples in the research paper included one collected in the 1840s, with a hammer, and a second dating from the 1920s, which was excavated at the site, as a bit of detritus. And it is those two slivers that led researchers to a conclusion about the geographic provenance of the Altar Stone.

“They are both fragments of the Altar Stone with the exact same characteristics,” Ixer said.

The analysis of the Altar Stone was also overseen by Chris Kirkland at the Timescales of Mineral Systems Group within Curtin’s School of Earth and Planetary Sciences. This cutting-edge laboratory usually spends its time helping to look for iron ore and gold deposits.

In this case, the group used lasers to light and mass spectrometers to explore the individual grains in the Altar Stone sliver, paying most attention to the minerals apatite, rutile and zircon, which decays at a known rate and so becomes a kind of “geologic clock” for dating material.

They found the specific grains in the Altar Stone contained minerals that were as old as 2 billion years and others 450 million years old, which produced a kind of chemical fingerprint.

The geologists from Curtin say with 95 percent certainty that the Altar Stone is Old Red Sandstone from the Orcadian Basin in northeast Scotland, an area that stretches from the Orkney Islands to Inverness.

Two other stones make up most of the Stonehenge assemblage. The big sarsen stones, which form the upright posts and lintels in the postcards – the dramatic image of the monument – were massive, but they came from nearby quarries, not too many miles away.

The others are called “bluestones,” which form the outer circle, and these stones came from quarries in Wales.

How all the sarsen and bluestones were moved remains a hotly debated topic – were they wrapped in willow baskets and rolled? Or pushed along on sleds, or rolled on logs, or just dragged by humans with rope and leather, maybe helped by oxen?

As for the Altar Stone, these researchers like a maritime route. They point out that neolithic people introduced the common vole from continental Europe to Orkney. There is also evidence of the long-distance marine transport of cattle and quarried stone tools and pottery. Goods and animals crossed the English Channel way back then.

Researchers have found evidence of neolithic dugout canoes – but mostly in rivers, not seaside. Seagoing rafts, perhaps boats with hulls lined with animal skins? That is a good guess, but there is little physical evidence on the shorelines because these kinds of materials decay quickly.

The geologists here say that more geologists will now work hard to pinpoint the exact place in Scotland where the Altar Stone came from – and then they confess that another generation of archaeologists will puzzle to figure out how and why the stone was moved, perhaps never to know.

“As these things may be lost to history,” as Clarke said. Or maybe not.