Silbury Hill is a Neolithic monument built around 2,400 BC - roughly at the same time as the great pyramids in Egypt. The hill stands around 40 metres high and has been calculated to contain around 250,000 cubic metres of earth; it has been described as the largest man-made mound in Europe. It forms part of the Stonehenge and Avebury UNESCO World Heritage site.
Investigations have taken place over the centuries and in 1776 a shaft and tunnels were excavated by Cornish Miners to investigate what lay within. In 2007, after further recording, these tunnels were back-filled with chalk by English Heritage and so access has now been lost.