are, trundling along this ordinary tarmac road through this high Wiltshire countryside, you turn a corner, and this great rock pops up in front of you, throwing you back four thousand years, in an instant.
Vaguely diamond-shaped, it stands some 14 feet high - maybe 10 feet wide, 3-4 feet thick. It's massive.
Although it's not finely sculpted or smoothed, there can be no doubt that this was chosen and placed by the hand of man - and not yesterday.
You don't need to be an archaeologist to be aware that this stone is very, very, very old.
But, if you're in a bit of a hurry, you might easily drive past this stone, not really noticing the other stones arrayed around the fields on either side of the road.
You might notice, casually, that Avebury is a pretty little village. As you slow down to negotiate the kink in the road, you might be tempted to stop at the Red Lion pub - it's certainly attractive.
As you leave the village, you might wonder, idly, about some oddly-shaped features in the landscape. But, by the time you encounter the traffic on the A4 at Beckhampton, you might easily be thinking of other things.
Don't feel guilty - people have been doing that for thousands of years.
In the modern era, Avebury was almost completely unknown until a Wiltshireman called John Aubrey came upon this stone and its companions whilst out hunting in the year of 1649.
He was "wonderfully surprised at the sight of these vast stones of which I had never heard before."
Aubrey was an antiquarian, with some experience of ancient things. More importantly, he had an imagination, and knew, almost immediately, that these stones, and the odd earth-shapes within which they stood, were something very special.
There wasn't much enthusiasm for ancient monuments under Oliver Cromwell's Protectorate, but the Restoration of the Monarchy (1660) brought to the throne a king who thirsted for knowledge, and was prepared to give his support to many scientific and cultural efforts.
Aubrey showed Charles II round the Avebury site in 1663 and in the same year, the first, rough maps of the site were shown at the Royal Society (of which Aubrey was a founder member).
Aubrey wrote that Avebury "does as much exceed in greatness the so renowned Stonehenge, as a cathedral doeth a parish church."
Odd, therefore, that Avebury has never really captured the British imagination in the way that the inferior Stonehenge has.