At 39 metres (about 130 feet) high, Silbury Hill is said to be the largest prehistoric artificial mound in Europe. © English Heritage and Skanska.

It’s one of prehistoric Britain’s most extravagant and imposing monuments, but Silbury Hill in Wiltshire is also amongst its most threatened. However, staff at English Heritage have reached a decision as to how they intend to safeguard its future.

Major concerns about the stability of the later Neolithic-era mound were raised by experts, campaign groups and the general public in 2000 when a partial collapse left a gaping hole at the top.

Following extensive surveying in 2001 and in 2003, as well as consultation with a range of interested parties, English Heritage - which is charged with the care and upkeep of the monument – has chosen its preferred option for shoring it up.

In a statement, English Heritage has confirmed that it will now explore the feasibility of tunnelling into the mound and shoring up what voids have been left by previous excavations.

A hole appeared at the top of the man-made hill after a period of heavy rain in 2000. Photo: Damian Grady. © English Heritage.

"The option chosen is to re-enter Silbury Hill via a tunnel dug to its centre in 1968, and then remove existing collapse and inadequate backfill in the tunnel, before properly backfilling it," reads the statement. "The tunnel and other voids within the hill would be filled with chalk to the same density as the surrounding mound material."

The backfilling will be undertaken - backwards from the centre of the hill - by specialists, who will be appointed by English Heritage once a detailed brief has been drawn up and the contract put out to tender.

It is also the intention to carry out an archaeological investigation programme to, according to the statement, "fully record all the parts of the hill which are exposed again and enhance our knowledge of its construction."

A report by a local newspaper suggests the work, in its entirety, will cost between £500,000 and £600,000.

The largest artificial prehistoric mound in Europe, Silbury Hill was built between 2800 and 2000BC (around the same time as the pyramids in Egypt) and has continually fascinated historians and archaeologists. As such, several attempts have been made to try and find out what, if anything, is inside.

Photo: Damian Grady. © English Heritage.

It seems that it is this quest for Silbury’s secret that has led to its weakening and the threat of a collapse.

In 2000 a hole appeared at the very top, where an excavation shaft was sunk under the direction of the Duke of Northumberland in 1776. It’s uncertain whether or not he found anything but further excavations in 1849 and 1968, which involved a second tunnel, failed to reveal why the hill was built.

However, English Heritage maintains that despite the weaknesses created by these excavations the vast mound is essentially safe from total collapse.

"In its work to date on the hill, English Heritage surveys have confirmed that the overall structure of the mound is stable," adds the statement, "although there are localised pockets of instability."