You're in your jouncing people-carrier, taking in the agreeable but unremarkable view, and then suddenly it's upon you; a pointy attention-grabber at the side of the road, towering street furniture in the shape of a hazard-warning equilateral. This is crushing historical time expressed in trigonometry.
Old Egypt hands could be forgiven for thinking that the terrible shark's fin that I'm talking about is the sort of thing that looms in your windshield as you're driving through the suburbs of Cairo.
But they'd be wrong. Or they'd be half-right. Silbury Hill is a pile of chalk just off the A4 in Wiltshire but it's been called "Europe's answer to the pyramids".
There's its silhouette, for a start. Then there's the fact that the tons of calcium carbonate which went to make it were being quarried at about the same time as the pharaonic tombs were going up.
Perhaps the most pertinent similarity is that no one knows exactly how Silbury was built.
Indeed, as you round a bend out of Marlborough and come face to face with the hill, the thought that our distant kin are credited with building something so utterly alien is what immediately connects it with the ziggurats of the Nile delta.
completed about 2350 BC
excavated in 1776, 1849, 1968
No wonder that it's said to have been a clod of earth which fell from the Devil's foot as he flew over the Wessex plains, or a landing beacon for UFOs.
Others maintain that it's the mausoleum of a long-lost monarch. King Sil (hence 'Sil-bury') and that he's interred with a statue of his faithful steed, cast entirely from gold.
In fact, no one's sure what they got up to at Silbury as much as 4,400 years ago, though the latest dig has come out strongly in favour of singing and against sacrifices.
On a bright morning not so long ago, motorists may have seen adepts of early music ascending the 40 metre hill with their horns and their fifes, their skins and their bladders, in order to establish that sound carried from the summit to the plains below.
The hill was a bandstand, at least for some of the time. Later, under Norman rule, it was a lookout post.
There are no signifiers of ritual killings, apparently. Silbury is a very clean site, in the jargon, though not as a clean as it must have been before turf grew over the chalk, when it might have looked from a distance like a mammoth's tusk.
Such are the results of the painstaking proddings and scrapings of English Heritage, who are responsible for the hill.
Earlier prospectors weren't so kid-gloved, sinking a mineshaft into the crown and hacking into the flanks. Locals piled in, hopeful of stumbling upon King Sil's 24-carat nag.
The effects of those clumsy fumblings became apparent a few years ago when a hole opened at the apex of the hill. The tunnelers had disturbed the chalk, creating voids or air holes.
A great English monument was in danger of falling in on itself like a sagging Yorkshire pudding.
We were the last civilians to walk inside the hill
One last excavation was sanctioned and a team from Newsnight joined it in its final stages.
The historians have uncovered arrowheads, and dense, stony antlers which were used as tools.
They now know that Silbury didn't go up all at once but in three stages. Deep inside the hill, beneath up to half a million tonnes of ever so slightly unstable chalk, we goggled at the perfectly preserved fronds of moss which had grown four millennia ago on the slopes of the original mound.
We were the last civilians to walk inside the hill, along its tunnels, beneath groaning beams that wouldn't have been out of place in a gold mine.
That's what impressionable or avaricious people thought Silbury was, of course, though the gilded gee-gee was never recovered, and the latest technology has failed to find any human remains, royal or otherwise.
English Heritage believe that the only way to conserve the hill is to close it up. Miners now labour below ground to pack the chalk and seal it with a special paste. In a few days or weeks, the grail of thousands of years will be beyond reach and Silbury - the great tepee, the big tent, of our woad-striped forefathers - will be restored to enigmatic silence.