Can there be anything more po-faced than a health and safety briefing?

We are about to enter some rather iffy tunnels dug into Silbury Hill, in Wiltshire, a 4,400-year-old man-made pyramid constructed entirely from chalk.

It will be wet, dark and nasty in there, so we need to take precautions. There is talk of wearing hard hats, stout footwear and highvisibility jackets.

But what makes our briefing slightly less catatonic than the average health and safety lecture is the presence of Terry Dobney.

He looks, well, different from the other officials.

For a start, he is wearing a long white robe.

And he sports facial hair of Biblical proportions.

He is carrying a piece of antler, and a long wooden staff with a crystal in the top.

Terry, you see, is an official site druid. But that doesn't mean he doesn't have to wear a hard hat like everyone else.

Mystery: Archdruid Terry Dobney offers his explanation for Silbury Hill

All the spirits of heaven and earth won't save him if one of those tunnels collapses on his head.

It seems appropriate that our expedition should be accompanied by one of the more bizarre manifestations of officialdom, because we are at one of the most extraordinary and impressive ancient monuments in Britain, if not the world.

The hill is actually a truncated grasscovered pyramid about 120ft high and more than 600ft across, weighing an estimated half a million tonnes.

Silbury, just south of Avebury, took more than 100 years to build and work began, according to the radiometric dating that has been carried out on bits of vegetation and other material recovered from its innards, about 4,400 years ago.

Over the centuries, various lunatics have riddled the ancient chalk construction with tunnels, mostly in an attempt to find treasure.

The latest was in 1968 and was at, quite extraordinarily, the behest of the BBC.

They hacked into this unique national treasure with impunity to make a series of gimmicky history programmes.

They even built a recording studio in there.