How? Who? Why? From Agatha Christie to Dan Brown, all the best thriller writers know the value of leaving these questions dangling until the last page. But at least we discover the answers in the end. If mankind endures ten million years we may never discover the who and why of the most tantalising enigma in the British landscape.

No, not Stonehenge. That jumbo-sized sundial may still retain an awesome aura of mystery, but its makers left dozens of clues as to its function. I’m talking about the vast monument a few miles north – Silbury Hill, the 500,000-tonne mound of chalk and turf that our Neolithic ancestors spent an estimated four million man-hours constructing, for reasons that have been totally obliterated by the march of millennia.

After 4,300 years, Silbury is in the news again. Last week experts from English Heritage, using pioneering tunnelling technology from the engineering company Skanska, reached its centre, 40 metres beneath the summit. Their primary purpose was not archaeology but preservation. Because of numerous botched explorations – from the Duke of Northumberland’s piratical attempt to dig for “hidden treasure” in 1776 to a BBC-funded dig in 1968 that was curtailed when nothing sensational (in TV terms) was discovered – the hill has developed alarming craters. So the present £1 million plan is to stabilise Silbury by backfilling the various tunnels and shafts with chalk.

But of course the archaeologists have also seized on this as the last chance to get to the heart of the mystery. We know that Silbury was built, perhaps by that strangely advanced bunch known as the Beaker People, in three phases over several centuries – at roughly the same time as Stonehenge and the stone circles at nearby Avebury. It was originally a straight-sided polyhedron with a domed top (the Saxons probably flattened that to make a fort during the Danish invasions). And only this year the remains of a Roman village were discovered at its foot.

But what was its purpose? One legend says that it was the burial mound of a mythical king called Zel (or Sil) and his horse, and that fabulous gold effigies of monarch and steed are buried at its base. Indeed, it was in the hope of finding such Sutton Hoo-style treasure that the BBC began its dig.

Silbury, however, was never a burial mound in the sense that its near neighbour, West Kennet Long Barrow, was. No human remains have ever been found there, let alone gold effigies. Could it, though, have been a tomb not for bodies, but souls? That remarkable theory is being advanced by English Heritage’s experts. They have found that Silbury’s builders buried hundreds of sarsen stones in the hill. And they maintain that sarsen, which was also used at Stonehenge and Avebury, had sacred significance for Ancient Britons and may well have been seen as enshrining the spirits of the dead.

Intriguing and plausible. But so are other theories. One, favoured by modern druids, is that Silbury was an altar for a water-related cult. The ditch around it, they argue, wasn’t simply a byproduct of digging up the chalk needed for the hill, but also a moat filled with water from the spring that is the source of the River Kennet, just 200 metres away. That would have created a magical illusion of a gleaming white island rising from a lake – not unlike the way the Lincoln Memorial is mirrored in the Reflecting Pool in Washington. How little our public monuments have changed in 4,000 years!

Another theory notes that Silbury could be seen from Avebury’s sacred sites only when the corn on the hills around it had been harvested. This suggests that its construction was connected with harvest and fertility rites. Perhaps the hill was an effigy of an earth goddess who needed plentiful homage to produce a decent harvest. That fits the historical evidence. After all, arable farming was the civilisation-transforming innovation of the Neolithic Age.

Much has been written, too, about Silbury’s position on the intersection of ancient ley-lines. Could its builders have been following their version of feng shui, harnessing the “earth energies” believed to have coursed through these prehistoric tracks?

But how would Neolithic Britons, who couldn’t even write, have devised such a complex psycho-geographical plan? There are theories to explain that, too. Perhaps the most eye-popping is expounded by John Cowie in his unintentionally hilarious book Silbury Dawning, which suggests that aliens landed 13,000 years ago and interbred with human beings, thus producing a huge leap in intelligence. Hmm. I think I prefer the old folklore explanation for Silbury’s creation: that it was a load of rubble that the Devil intended to drop on the townsfolk of Marlborough (what they had done to incur his wrath isn’t recorded), before a quick-witted cobbler persuaded him that Marlborough was too far away to reach on a warm day.

What’s clear is that this little corner of Wiltshire – a sacred landscape for thousands of years – holds as many prehistoric secrets as the whole of Egypt. Indeed, a current Sheffield University dig close to Stonehenge is rumoured to be uncovering evidence of funeral rituals that predate the henge itself by centuries if not millennia.

Very exciting. But I hope the diggers don’t come up with too many “solutions” to the mystery of Silbury and its inscrutable entourage of henges and barrows. To gaze on that prodigious pimple, especially with a mist swirling over the downs, is to feel both the ambition and the vainglory of Homo sapiens. All that effort, planning and ingenuity, gone into producing a monument to a belief, a person, a regime, an event, that must have seemed frightfully important at the time, yet is now totally erased from human chronicle and consciousness. Few other places on the planet induce so profound a sense of mankind’s microscopic insignificance when measured on cosmic timescales.

Perhaps that’s what those aliens were trying to tell us, 13,000 years ago.

Bong! Give Julie’s gaffes a green light

Can’t wait to see ITN’s resurrected News at Ten. Not because I have a thing about hunky Sir Trevor McDonald – that was a long time ago, and I’ve had treatment – but because it will be co-presented by Julie “whoops, my mic is still on” Etchingham. Gorgeous but careless Julie, you will recall, was the Sky News presenter who, forgetting that she was still on air, quipped “extermination” as David Cameron outlined Tory policy on immigrants.

Far from apologising for the “gaffe”, TV news producers should be seizing on it as a brilliant new way of holding the viewers’ attention during dreary political coverage. Imagine what fun it would be if we could press the red button on our remote controls and get a candid running commentary on what these po-faced presenters are thinking as they hear slippery politicians spouting their evasive platitudes.

Only one thing would be more entertaining: a direct line to what the politicians themselves are really thinking as they spin us their preposterous soundbites.

Glummy dummy

Why do fashion models have to look so blinking miserable? There was a lass in The Times Magazine on Saturday kitted out in a £2,485 tweed jacket (silly girl, she could have had my old tweed jacket for ten quid), £117 jeans, £265 boots, £150 sunglasses, £2,050 “love bangles” (whatever they are), a £9,200 Rolex watch and about £250 worth of sundry bits and pieces. Total cost of outfit? Something to the north of £14,500, if my creaky arithmetic is correct. But could she muster a smile as she flaunted more in bling and fabric than some folk earn in a year? Could she hell! She looked as if she had just caught a whiff of something rank drifting from a blocked drain.

Which, of course, reminds us chaps of the First Law of Human Relationships. There’s no pleasing a woman.

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