Silbury Hill was constructed in several stages during the late Neolithic period. The base is circular, 167 metres in diameter, and the summit is flat-topped at 30 metres across. A smaller initial mound was built first, consisting of a gravel core with a kerb of stakes and sarsen boulders, then progressively enlarged with alternate layers of chalk rubble and earth. The material was excavated from a series of surrounding ditches that were dug, filled, and recut further outward as the mound grew. The precision is remarkable: the centre of the flat top and the centre of the cone lie within a metre of each other. Yet archaeologists Jim Leary and David Field, who studied the mound extensively, concluded that there was no master plan -- the multiple overlapping construction phases suggest almost continuous remodelling, as though the process of building mattered more than the finished product.

Silbury Hill has drawn investigators since antiquity. The Roman road from London to Bath swerves around it, proof that the hill predates the road and was too significant to bulldoze. The seventeenth-century antiquarian John Aubrey illustrated it. William Stukeley recorded a skeleton and bridle found during tree planting on the summit in 1723. In 1776, the Duke of Northumberland hired Cornish miners to sink a vertical shaft from the top -- they found nothing. In 1849, a horizontal tunnel was dug from the edge to the centre -- again, nothing of obvious significance. The Wiltshire Archaeological and Natural History Society excavated in 1867, and Professor Richard Atkinson brought BBC Television cameras to the site between 1968 and 1970, revealing environmental evidence but no definitive purpose.

In May 2002, heavy rain caused the 1776 shaft to collapse, opening a hole in the summit. English Heritage undertook seismic surveys and discovered that the previous excavations had left the hill honeycombed with unstable voids. A major stabilisation programme began in 2007: contractors filled the old tunnels and shafts with hundreds of tonnes of chalk while archaeologists conducted a new survey with modern equipment. The work confirmed that the mound dated convincingly to the Late Neolithic. In 2007, English Heritage also announced the discovery of a Roman village at the foot of the hill, with regularly laid streets and houses -- evidence that the mound continued to attract settlement long after its builders had vanished.

In the absence of explanation, stories have filled the void. Local legend holds that Silbury is the resting place of a King Sil, sitting on a golden horse in a golden suit of armour. Another tale, recorded in 1913, says the Devil was carrying a bag of soil to dump on the citizens of Marlborough when the priests of Avebury stopped him, and the dropped bag became the hill. Hundreds of people from surrounding villages once gathered on Silbury's summit every Palm Sunday. Modern theories range from a ceremonial platform elevating a priesthood above the surrounding populace to a component of an intervisible landscape of sightlines connecting Avebury, West Kennet Long Barrow, the Sanctuary, and Windmill Hill. Perhaps the most honest assessment comes from Leary and Field: the purpose cannot be known. The hill is its own answer.