The souls of Silbury Hill are bared in burial mound dig

Archaeologists are unlocking the secrets of Silbury Hill, 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.

Stones have been seen by many cultures as spiritually and physically interchangeable with humans – with a belief that particular stones contained the souls, spirits or even the transformed mortal remains of the dead. The belief was widespread, occurring all over the world.

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 2400BC, 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.

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.