Archaeologists are unlocking the secrets of Silbury Hill, Europe's tallest man-made mound and one of Britain's greatest historical mysteries. Researchers have long been mystified as to why the giant prehistoric mound in Wiltshire was built. But following one of the UK's most extensive and expensive digs, they appear to have found their answer: Silbury Hill may well have been a tomb, not for bodies, but for the souls of the dead.
     The English Heritage dig, which cost £1m, tunnelled 85 metres into the 40-metre-high man-made hill, discovering that its Neolithic builders had incorporated hundreds of heavy sarsen stones into its matrix. Sarsen, the silicified sandstone still found in great quantities in Wiltshire, was also used to build Stonehenge and Avebury. Heavier than other types of stone, archaeologists have long suspected that the material was regarded as sacred by Neolithic man.
     Silbury Hill, researchers believe, could well have been built as a sort of spiritual tomb, filled with spirits rather than skeletons.
"The new information we are obtaining from inside Silbury Hill is transforming our understanding of the site," said the English Heritage archaeologist Jim Leary, who led the three- year investigation. "The discovery of sarsen stones inside the final phase of the monument has also been a surprise. Given the almost certainly religious and ceremonial nature of Silbury, it is likely that these stones had some symbolic importance, potentially representing the spirits of dead ancestors."
     Radio-carbon tests on the mound have also revealed the age of Silbury Hill for the first time. Archaeologists now believe construction on the primary mound started about 2400 BCE, which would mean it was built at the same time as Avebury and the first phase of Stonehenge. Also revealed for the first time is the probable original shape and size of the monument. Excavations at its summit suggest it had a rounded rather than flat top and was five to seven metres higher than today, having almost certainly been flattened in late Saxon or Norman times to accommodate a wooden fortress.
     Leary believes that Silbury, and monuments such as nearby Stonehenge and the stones at Avebury, were built in response to a period of great change in Britain, which at the time was being influenced by an influx of European cultures. It does not appear anyone lived there in the Neolithic period, perhaps because it was too sacred. Experts are also now convinced that Silbury Hill was constructed in three separate stages. Silbury One - which Mr Leary identified in the heart of the hill - was a 15ft high, cone-like stack of turf capped with clay in 2400 BCE. Silbury Two was built of piled rubble chalk on top of the original monument almost immediately afterwards. Archaeologists believe there was a gap of a few hundred years before Silbury Three was constructed on top of Silbury Two - possibly in about 2000 BCE.
     Researchers also now believe Silbury was associated with a form of river-related religious cult. Until the 19th century, the linkage between the Kennet river and Silbury was reflected by an annual local ritual in which water was collected from the main source of the river – the Swallowhead Spring, 200 metres from the monument – before being taken to the top of Silbury where it was mixed with sugar and then drunk.
     The current excavation has also allowed Jill Campbell, an EH archaeo-botanist, to uncover what she claims is the first fully preserved mature chalk grassland complete with seeds, ants, moss, beetles and grass. "The preservation conditions are unique," she said. "We can tell it was grazed chalk land. It is evidence that the Neolithic people here managed and farmed this landscape.
     Heavy rains in 2000 led to the collapse of a top-to-bottom excavation shaft commissioned by the Duke of Northumberland in 1776. As a temporary fix, EH plugged the hole in 2001 with polystyrene to stave off further sinkage. Later this year they will begin removing all steel work from inside the mound before in-filling all the crumbling internal cavities with chalk - 1,000 tonnes lie crushed and ready to be pumped in when the time comes. The aim is to seal it up by Christmas forever so no further damage is done. In the meanwhile, archaeologists have a limited time to scour the inside of the mound.

Sources: Daily Mail, Guardian Unlimited, Western Daily Press (25 October 2007), The Independent (28 October 2007)