BACKGROUND: Problems with the hill, a Neolithic monument, were revealed in 2000 when a hole appeared in its summit above a shaft originally sunk in 1776. The hole has been temporarily capped but stabilisation work is needed at the hill's summit and inside a 1960s tunnel.
WHO IS BEHIND IT? English Heritage and Skanska.
PROJECT AIMS: To save the hill from collapse and find out as much as possible about its mysteries before the tunnels are sealed.
SKILLS INVOLVED: Conservation, partnership working, engineering and archaeology.
Silbury Hill in Wiltshire is the largest artificial prehistoric mound in Europe. But the activities of treasure hunters and archaeologists have left it in danger of collapse.
A £1 million conservation scheme is combining engineering with archaeology so researchers can try to uncover more about the hill's mysterious past before the tunnels are sealed and the 4,400-year-old Neolithic monument is stabilised.
English Heritage archaeologists and engineers from construction firm Skanska have reached the centre of the hill through a tunnel originally sunk into its side in the late 1960s by the archaeologist Richard Atkinson.
However, efforts proved especially challenging after the summer's heavy rains made tunnelling more dangerous and the old Atkinson tunnel collapsed because of heavily saturated clay in the central chamber. A tunnelling method had to be devised allowing excavations to known voids in the hill's centre to continue. This involved replacing Atkinson's arches with more secure mining supports.
The scheme has already thrown fresh light on the use and design of the mound. Archaeologists have discovered a series of medieval postholes on top of the hill and two iron arrowheads and now think that today's iconic shape is very different to that of the monument when it was first built. They suggest that the summit changed from what was originally a dome into a flattened top hundreds of years later.
"We believe that the top of the hill was literally lopped off around the time of the Battle of Hastings, or even earlier when the Danes attacked in 1006, to create flat land for a military base," explains English Heritage prehistorian and archaeologist Jim Leary. "The absence of Roman deposits and Atkinson's discovery of 11th and 12th century pottery in the side of the hill seem to support the theory that there was a fortified Saxon or Norman building on the summit."
Archaeologists are now recording and investigating the 85m long tunnel in detail. It reaches right to the heart of the hill nearly 40m below the summit under thousands of tonnes of chalk and cuts through each of the three main elements of the construction, which experts believe were built separately.
Silbury I, the oldest part of the hill, was constructed by Neolithic builders as a stack of turf with a capping of clay. Silbury II was built of piled rubble chalk very soon afterwards, in around 2400 BC. Archaeologists believe that there was then a gap of a few hundred years between the building of Silbury II and Silbury III. English Heritage hopes to create a complete picture of the Neolithic landscape including vegetation, climate and how the land was managed by prehistoric people for grazing and arable and woodland use.
"Until now, the best guess was that Silbury Hill was constructed over anything between 100 and 500 years," says English Heritage chief executive Simon Thurley. "When our archaeologists and dating team have done more precise work we will be able to narrow this estimate and understand better how and why this monument was built.
"A shorter construction period over a couple of generations might indicate that it was a heroic piece of work led by one or two charismatic individuals. If it stretches over hundreds of years and many generations, we can conclude that it was integral to a much more long-standing set of spiritual beliefs."