When Silbury started caving in on itself a few years ago, a plan was hatched to repair the damage by pouring in hundreds of tons of chalk paste to fill the voids.

By the start of next year Silbury should be safe, its voids filled, making now about the last time anyone will be able to venture down the tunnels.

We enter the hill through a portal - constructed for the BBC team 39 years ago.

Inside it is dank, dark, wet underfoot and, frankly, rather horrible. Overhead, keeping up the thousands of tons of loosely-compacted chalk, are steel arches and some badly rusted pit props.

Above, on a couple of occasions, we see chutes leading up into the gloom, the remains of the exploratory shafts dug in the 18th and 19th centuries.

On the walls are numbers on pieces of paper, which mark the places where significant strata have been identified.

According to chief archaeologist Jim Leary, the construction technique was "very impressive", with carefully packed chalk rubble shored up by stone facing.

I am shown a layer of clay.

"This was the orginal surface, the ground," says Jim.

An indentation shows where - perhaps - the heel of a human foot struck the ground more than 4,000 years ago.

I try to imagine what this sliver of Wiltshire was like back then.

The same rocks, the same hills, but subtly different; more trees, possibly. Warmer, definitely, maybe two degrees more so than today; and in the woods, wolves and bears, the indigenous large carnivores of southern Britain.

Humans were not necessarily the top of the food chain back then.

This really is a trip into the bowels of England's past.

Back outside, after a steep, wheezy climb to the top, one can see the various other wonders of Neolithic Wiltshire: the stone circle at Avebury, together with a long avenue of standing stones; the nearby West Kennet Long Barrow and, 16 miles over the horizon to the south, Stonehenge.

It is a magical place.

The questions are, of course, who built all this stuff and why?

The first question is more easily answered than the latter.

Modern genetic analysis reveals that after the ice-age glaciers receded from Britain about 8-10,000 years ago, these islands were repopulated by probably two waves of peoples - one from what is now Spain and Portugal, the other from the north and east, from what is now Denmark and Norway.

And it is from these peoples, who built Silbury and Stonehenge, that most British people today are descended.

But as to why our ancestors built these huge monuments, we can only guess. Stonehenge was certainly some sort of astronomical instrument - and the various standing stones not only of Wiltshire but further west in Cornwall and to the far south in Brittany, France, may similarly have served some sort of astronomical purpose.