There's a lot that we think we know about Stonehenge. We're almost certain, for example, that the great prehistoric monument was built in several phases spanning hundreds of years, from around 3000 BC to 1600 BC. We know, too, that it was a construction project that tested ancient ingenuity and prehistoric technology to the limit.

And given the time and effort involved, as well as the scale of the ambition, we can be pretty confident that Stonehenge was one of the most significant points on the landscape of late Neolithic Europe.

But what we don't know is perhaps the most important question of all. Archaeologists have gone some way in answering the 'how', 'what' and 'when' of Stonehenge (and indeed, the Timewatch dig will attempt to narrow down the 'when' even further, by dating precisely the arrival of the bluestones). But they're still some way from a definitive answer to the question 'why?' Four thousand years and more after Stonehenge was built, nobody is really sure what it was built for.

That might be about to change. Two of Britain's leading archaeologists, both world-renowned experts on Stonehenge, think they may have finally solved the riddle of the great standing stones. Professor Timothy Darvill and Professor Geoff Wainwright (pictured) are not convinced, as others have been, that Stonehenge was a holy place or a secular tool for calculating dates. Instead, they think Stonehenge was a site of healing.

"The whole purpose of Stonehenge is that it was a prehistoric Lourdes," says Wainwright. "People came here to be made well."

This is revolutionary stuff, and it comes from a reinterpretation of the stones of the henge and the bones buried nearby. Darvill and Wainwright believe the smaller bluestones in the centre of the circle, rather than the huge sarsen stones on the perimeter, hold the key to the purpose of Stonehenge. The bluestones were dragged 250km from the mountains of southwest Wales using Stone Age technology. That's some journey, and there must have been a very good reason for attempting it. Darvill and Wainwright believe the reason was the magical, healing powers imbued in the stones by their proximity to traditional healing springs.

The bones that have been excavated from around Stonehenge appear to back the theory up. "There's an amazing and unnatural concentration of skeletal trauma in the bones that were dug up around Stonehenge," says Darvill. "This was a place of pilgrimage for people...coming to get healed."

So the ill and injured travelled to Stonehenge because the healing stones offered a final hope of a miracle cure or relief from insufferable pain. But though Darvill and Wainwright think the idea of Stonehenge as a prehistoric Lourdes is the most convincing yet, it's fair to say that the archaeological community is not completely convinced. When the theory was first proposed at a talk in London in 2006, it was met with considerable support, but also one or two dropped jaws.

And that's not surprising. A consensus among archaeologists on what Stonehenge was actually for has proved as difficult to build as the huge stone circle itself. There have been plenty of theories. One is that the great stone circle was a gigantic calendar. Put simply, the site's alignment allows for the observation of astronomical events such as the summer and winter solstice. With that information, our ancient ancestors could establish exactly where they were in the cycle of the seasons and when the site would be at its most potent.

But would they really have put so much time and effort into the construction of something that today we take for granted? Some archaeologists believed they would. Stonehenge offered a way to establish calendar dates when no other method existed. Accurate dating allowed for more efficient and successful agriculture, as well as the marking of important religious and social events.

But the most popular theory about the purpose of Stonehenge has survived since serious archaeological work first began on the site hundreds of years ago. The great standing stones, thrusting heaven-wards from the ancient plain, certainly inspire a religious reverence. Working in the early eighteenth century, William Stukeley was one of the great pioneers of archaeology at Stonehenge. He was struck by its innate spirituality.

"When you enter the building..." he wrote in the early 1720s, "and cast your eyes around, upon the yawning ruins, you are struck into an exstatic [sic] reverie, which none can describe."

Many since Stukeley have also felt the power of the 'yawning ruins', and come to the conclusion that Stonehenge was a place of worship. Most recently, a project lead by Professor Michael Parker Pearson of the University of Sheffield has attempted to place Stonehenge in a wider landscape of religious ceremony.

His interpretation is at odds with that of Darvill and Wainwright. Stonehenge was not a place for the living, whether sickening or fighting fit. It was a monument for the dead. According to Parker Pearson, "Stonehenge... was built not for the transitory living but for the ancestors whose permanence was materialised in stone."

An even more remarkable origin is suggested by other theories of Stonehenge. To some in the excitable 1970s, Stonehenge was a landing pad for extraterrestrial visitors. It's fair to say that the archaeological evidence for this - laser guns and jetpacks perhaps - has yet to be unearthed.

Modern technology has allowed us to discredit some early explanations of Stonehenge's purpose, however. We know that Stonehenge was not a Roman temple, and accurate dating has also shown that it was completed at least a thousand years before the Druids roamed the British Isles. The notion of Stonehenge as a prehistoric Lourdes appears to be more compelling, but we will have to wait and see if the latest in a long line of theories will finally solve one of history's most enduring riddles.

Published: March 2008

c The BBC ~history