Silbury Hill: dug into three times, and empty

Silbury Hill stands about thirty metres high and covers roughly two hectares at the base. It is the largest artificial prehistoric mound in Europe and was built in the Avebury landscape around 2400 BC.

The volume of material moved is estimated in the hundreds of thousands of cubic metres, all of it chalk and turf, all of it cut and carried by hand.

The three digs

In 1776 the Duke of Northumberland employed Cornish miners to sink a shaft from the summit. They found nothing.

In 1849 a tunnel was driven in from the side. Nothing again.

In 1968 to 1970 Richard Atkinson excavated with BBC cameras present, on the expectation of finding a burial. He did not. The programme had to report an absence.

What the stabilisation found

That it was built in stages, over generations, rather than in one campaign. Successive enlargements are visible in the structure, each apparently complete in itself before being enlarged.

That there is no chamber and no burial. The centre contains gravel, soil, turf and chalk. This is now about as certain as any negative finding in British archaeology.

And that the process of building mattered. The layering suggests deliberate, repeated construction rather than mere accumulation.

What building it actually cost

Estimates of the labour have been made from the volume of material and the rates at which people can dig and carry chalk.

The figures usually given run to millions of worker-hours — on the order of hundreds of people working for years, or a far larger number for a shorter period, and either way a sustained commitment by a society without draught animals, metal tools or the wheel.

The staged construction changes how that should be read. It was not one enormous effort but a series of them, separated by intervals, each apparently finished before the next began.

The Roman village at its foot

Geophysical survey and excavation have identified a substantial Roman settlement immediately around the base of the mound, occupied for centuries. The Romans did not build it and appear to have regarded it as significant. A monument already two and a half thousand years old when they arrived was still drawing people to it, which is a fact about its afterlife rather than its purpose.

The insects

The most precise thing known about the site comes from the turf at its base.

Preserved in it were winged ants. Flying ants swarm in a narrow window in late summer. Their presence dates the start of construction to late July or August of a particular year, four and a half thousand years ago.

That is the sort of detail that makes prehistory feel briefly close: a specific fortnight, in a specific summer, when people began to build.