At 37 metres high, Silbury Hill – which is part of the complex of Neolithic monuments around Avebury, which includes the Avebury Ring and West Kennett Long Barrow – is the tallest prehistoric human-made mound in Europe and one of the largest in the world; it is similar in size to some of the smaller Egyptian pyramids. Its original purpose however, is still highly debated. Composed mainly of chalk and clay excavated from the surrounding area, the mound covers about 5 acres. It is a display of immense technical skill and prolonged control over labour and resources. Archaeologists calculate that Silbury Hill was built about 4750 years ago and that it took 18 million man-hours, or 500 men working for 15 years to deposit and shape 248,000 cubic metres of earth and fill on top of a natural hill. The base of the hill is circular and 167 metres in diameter. The summit is flat-topped and 30 metres in diameter. A smaller mound was constructed first, and in a later phase much enlarged. The initial structures at the base of the hill were perfectly circular: surveying reveals that the centre of the flat top and the centre of the cone that describes the hill lie within a metre of one another.

There are indications that the top originally had a rounded profile, but this was flattened in the medieval period to provide a base for a building, perhaps with a defensive purpose. The first phase, carbon-dated to 2400 ±50 BC, consisted of a gravel core with a revetting kerb of stakes and sarsen boulders. Alternate layers of chalk rubble and earth were placed on top of this: the second phase involved heaping further chalk on top of the core, using material excavated from an encircling ditch. At some stage during this process, the ditch was backfilled and work was concentrated on increasing the size of the mound to its final height, using material from elsewhere. The step surrounding the summit dates from this phase of construction, constructed either as a precaution against slippage, or as the remnants of a spiral path ascending from the base, used during construction to raise materials and later as a processional route.  Few prehistoric artifacts have ever been found on Silbury Hill: at its core there is only clay, flints, turf, moss, topsoil, gravel, freshwater shells, mistletoe, oak, hazel, sarsen stones, ox bones, and antler tines. Roman and medieval items have been found on and around the site since the nineteenth century. Evidence exists that the hill was reoccupied by later peoples.