It is possible that it started out as a stone circle with an inner ring that, over time, grew into a mound. Stones were used to support the edges and this provided both the base and the core for future growth. According to various sources the initial mound was approximately 16ft high.
Archaeologists are uncertain as to the total construction period but it was probably measured in decades if not longer. The second phase seems to have involved piling chalk and clay on top of the central elevation from a ditch that would have encircled the central feature. As the mound continued to grow the ditch was backfilled and the overall effort seems to have focused on achieving both the size and shape that the hill is today. research seems to show that each level was buttressed with stones to ensure that the sides did not collapse. This is very similar to the techniques used in the construction of stepped-pyramids.
Although originally a prehistoric construction - Silbury Hill has clearly been used by successive generations. Proof of this is in the variety of minor artefacts that have been discovered from periods throughout the past few thousand years.
Several excavations of Silbury Hill have been carried out over the centuries. It was first recorded by the renowned antiquarian John Aubrey, who not only made notes regarding the structure but also drew a sketch of the hill. These were published as part of his work on the Monuments of Britain between 1680 and 1682.
The discovery of a buried skeleton and the reigns of a bridle were recorded by William Stukeley with reference to a tree planting that took place on top of the hill during 1723. It's generally believed that this burial dates from long after the hill was completed and its original purpose forgotten. However, this find may have been the catalyst, or at least added some weight, to the legend of King Sil. This folklore tale claims that within the mound was buried a golden statue of the ancient ruler and his horse. Therefore, as a word, Silbury could derive from the words 'Sil' and' beorg' (burial mound). No trace of this treasure has ever been found nor do serious archaeologists believe that it exists.
During 1776 a vertical shaft was sunk from the summit under the supervision of Colonel Edward Drax and the Duke of Northumberland. A second tunnel was excavated horizontally from the side to the centre in 1849. Neither of these two endeavours recorded finding anything of major significance.
The truth is that very little is known about this prehistoric hill. Researchers do believe that it was started during the month of August due to the discovery of specific winged insects but even this may be subject to debate as there has almost certainly been some climate change over the past millennia.
The hill itself has suffered meaningful damage from the random excavations that took place over the years and extensive work has been carried out to stabilise the sides and centre. It is no longer possible for walkers to climb the hill due to the damage caused by erosion.
There is obviously speculation as to why the hill was built and the importance of its location near to the Avebury Stone Circle as well as other ancient monuments. But ... even today the mystery remains and nobody knows for sure.