WITH his trusty wooden staff, flowing white robes and bushy, chest-length grey beard there is a definite whiff of Gandalf about Terry Dobney as he contemplates from this magnificent vantage point the surrounding Wiltshire countryside in all its pomp and glory.

Fields of green and gold stretch in all directions beneath a hazy blue sky: Swindon, seven miles to the north, is a blur on the horizon while the remnants of the world’s largest prehistoric stone circle are quite a sight several hundred yards away.

From a pouch Terry – the Arch Druid of Avebury – fishes out an assortment of grass and wildflower seeds and scatters them into the blustery wind. Moments later he is brandishing an antler pick while waxing lyrical about the monumental edifice on which a small group of us are standing.

“They used tools like this to build this staggering structure,” he says, waving the pick around before peering down the steep, grassy slope and adding, possibly with a sneer, “not shiny yellow JCBs like those just there.”

There is something deeply satisfying about all of this. A simple ceremony, symbolising re-birth and revitalisation, is performed on a glorious morning in May – exactly five years ago, give or take a day – to mark the completion of a truly unique project.

After eight years and £1.7 million an unparalleled, complex, modern-day feat of civil engineering has been undertaken to save an unparalleled, complex, prehistoric feat of civil engineering.

We are standing on top of one of the country’s – you could even say without exaggeration one of the world’s – most bizarre and mysterious buildings. It is, of course, Silbury Hill; not a hill in the accepted, natural sense but a man-made mound that has confused and confounded experts for centuries, as it continues to do today.

The initiative, which went about £1 million over budget but was clearly worth every penny, maintained for the foreseeable future the stability and integrity of a structure that has been described as Europe’s answer to Cheops’ Great Pyramid at Giza.

To have such a momentous spectacle of this age and calibre, a 20 or so minute drive into the countryside from Swindon, is something of which to be heartily proud.

During the early months of 2013, however, heavy rainfall has prompted new concerns for 130ft high Silbury Hill.

Unable to withstand the urge, trespassers have been shinning over the perimeter fence and clambering up the rain-soaked structure, causing “spectacular” damage, according to guardians of the hill, English Heritage.

“Some really rather hideous scars” have been created on the chalky banks of the artificial, multi-layered mountain, says archaeologist Jim Leary.

He is now urging visitors to admire this phenomenal structure from the official vantage points – just like the Pyramids – rather than cave in to the temptation of scaling the Neolithic pile, where public access has been prohibited for well over 30 years.

Like a lot of people new to the area, I can clearly recall seeing Silbury Hill for the first time. It was 1977 and to be honest I was completely unaware of its existence.

Admiring the famous, time-ravaged stones at Avebury while driving from Swindon to Devizes this weird hillock appeared into view is if from nowhere, a gigantic and outrageously un-natural lump in countryside. Over the decades many an unsuspecting motorist using the old London to Bath Roman road, the A4, must have gasped at it in amazement, their eyes compelled to wander from the highway, and exclaimed either verbally or by thought: “what the hell is that?” It evokes a genuine sense of disbelief and awe. Our prehistoric ancestors probably had no idea they were creating a 20th Century traffic hazard.

Built in several stages over many decades – nobody knows exactly why – Silbury Hill has dominated this archaeologically rich swathe of Wiltshire for more than 4,000 years.

There have been some imaginative and often fanciful theories for its existence; it is a Pagan tribute to Mother Earth; a prodigious sun dial; a spiralling ceremonial way; a landing pad for UFOs.

My favourite is that it is the resting place of the memorably named King Sil; not only that, but the mythical monarch was said to have been laid to rest, pharaoh-style, alongside some seriously impressive treasure… notably a golden statue of his good self, riding a horse.

Rumours of such gleaming wonders “from the bowels of the mountain” prompted, over the past 250 years, numerous excavations into the five-and-a-half acre mound, (about the size of Trafalgar Square.) The Duke of Northumberland in 1776 commissioned Somerset miners to sink a 100ft vertical shaft into the depths of Silbury from the top. The Dean of Hereford John Merewether’s team approached it from a different angle in 1849, burrowing a horizontal tunnel. In 1968, during the days of grainy, black and white television, millions of BBC2 viewers were glued to their sets as a young Magnus Magnusson commentated in excited tones on Professor Richard Atkinson’s excavation. Anticipating “a royal burial of exceptional richness” the nation held its breath as mining engineer Dr John Taylor broke into what was at first believed to be a burial chamber.

It turned out, however, to be Merewether’s 120 year-old shaft, along which Taylor gamely crawled into the heart of Silbury Hill. The gold, the silver? Not a shimmer.

Shamefully, none of these tunnels were properly back-filled and in 2000 a cavernous gaping hole, 18ft wide, 14ft deep, emerged at the summit – the result of numerous cavities and voids created over the decades by treasure hunters and exacerbated by rainfall.

Silbury Hill, Europe’s largest man-made prehistoric structure, had become “a worm-holed time-bomb waiting to implode.”

Thus followed, as English Heritage described it, “many years of meticulous research and planning.”

Using seismic technology, experts created a three dimensional computerised version of the hummock, complete with dents, shafts, fissures and other imperfections.

The plan? To tightly re-pack every void and tunnel with chalk in order to secure its shape and stability for centuries to come. It also involved re-excavating Atkinson’s 40 year-old tunnel in order to properly fill it in – and they kindly invited some press to snoop around before sealing the shaft forever.

So here we are, a handful of journalists on a biting October morning in 2007 about to venture to the very epicentre the mound. Exciting or what!

Sporting hard-hats and equipped with emergency breathing gear we make our way through the dank 278ft tunnel which is like a mine-shaft but nevertheless evokes childhood memories, at least with me, of Jules Verne’s Journey to the Centre of the Earth.

King Sil’s golden statue, alas, is nowhere to be seen. But at the very hub of Silbury Hill archaeologists have found treasure of a different kind: a 15ft high cone-like stack of turf… the original Silbury monument erected some 4,400 years ago.

Over subsequent decades our ancestors, with a fervour and purpose we can only guess at, built this modest structure higher and wider until they ended-up with a prehistoric cathedral larger than anything ever seen in Europe.

And they did it, countless thousands of them, by hacking the chalky earth from surrounding countryside with antler picks (parts of which were found during 2007 dig) and transporting it to the site in wicker baskets.

Seven months later, in May 2008, we are back to mark the grand conclusion of the project that will “preserve the ancient artificial mound for future generations.”

Staff in hand, Terry, who is in his late 50s, sets a fair old pace as he forges purposefully up the hill, a man on a mission.

He is invited by English Heritage to “sow the first seeds” of grass and wildflowers such as round-headed rampion, devil’s bit scabious, saw wort and horseshoe vetch that will help restore Silbury Hill’s good looks and fabric.

The brooding, enigmatic landmark is in fine fettle once more.

It is no longer, remarks the Arch Druid drily, in danger of collapsing into itself and becoming the Silbury Doughnut.