It is exactly 130 steps to the Neolithic heart of Silbury Hill, Britain’s most enigmatic prehistoric monument. A tunnel is giving archaeologists their last chance for at least a generation to study from the inside the largest man-made mound in Europe.

They have found a perfectly preserved section of prehistoric landscape, with grass that is still green and fresh mosses. It is not the gold that legend claims is hidden at the heart of the hill, but to the archaeologists it is even more valuable.

The people who built Silbury Hill quarried 500,000 tonnes of chalk and piled it up in a mound at least 40 metres (130ft) high and which covers an area of five acres. The hill stands beside the A4 at the heart of a landscape that includes Stonehenge and the Avebury stone circle a few miles away. Of the three it is regarded as the most mysterious, giving no clues as to who built it, or why. When it was completed it would have been a gleaming chalk beacon visible for miles around.

The opportunity to burrow to its centre arose because of an excavation that was broadcast live on television in the 1960s. Interest waned when it became clear that there was no hidden treasure, and the 85-metre long tunnel was hastily filled with road chippings on the orders of the BBC.

Over the next 40 years Silbury Hill, which was already suffering from the effects of three centuries of tunnelling by treasure hunters, began to collapse from the inside. English Heritage took the decision to dig out the tunnel created by the BBC and pack it with chalk to prevent further collapses. As professional tunnellers shored up the rusty iron girders, a team of archaeologists looked for clues that the original excavation missed.

At the mound’s centre is a layer of turf and gravel that preserved the only surviving prehistoric chalkland landscape beneath it and created another mystery. Study has revealed that the turf was carried from locations a considerable distance away.

Jim Leary, who leads the archaeological team, said: “There may have been some ritual significance, but there must have been a reason why people would carry it here rather than using what was available on the site.”

Radiocarbon dating has placed the construction of Silbury on the eve of the Bronze Age, a time of rapid social change when stone tools were starting to give way to metal. This coincides with big phases of construction at Stonehenge and Avebury as well as other ancient monuments now protected as a World Heritage Site.

Terry Dobney, a Druid, said: “It is a sacred mound, an effigy to the element of water built on top of a vast underground lake that feeds the Kennet and the Thames rivers. This would have been a place of tremendous significance in prehistoric times when people’s lives relied on water.”

Silbury Hill

2400BC First phase starts with turf mound

2400BC Silbury II, one of the chalk sections, is built, reaching 25m high

2000BC Silbury III completed: 40m high

AD300 Roman settlement on road between London and Bath c1066. Summit of Silbury Hill levelled off and fortified

1776 Colonel Drax digs shaft from summit in search of gold

1849 Royal Archaeological Institute commissions Dean Merewether to tunnel from the side

1968 Professor Richard Atkinson leads excavation broadcast by BBC

1986 Designated part of Avebury World Heritage Site

2000 Top of Colonel Drax’s shaft collapses

2002 Seismic survey finds large voids inside

2007 English Heritage spends £1 million stabilising hill

Source: English Heritage

Times Recommends

• Donor organ allowed child’s own heart to heal
• Michael Jackson: Not the final chapter
• Brüno toned down for 15 rating