Silbury Hill, near Avebury, Wiltshire, 2018. Dating from the late Neolithic period, Silbury Hill is the largest man-made mound in Europe. The 37 metre high mound has been subjected to several excavations. In 1776-1779 a shaft was opened at the summit, and an attempt to tunnel to the centre horizontally was begun in 1849. What these and subsequent investigations show is that the hill is constructed of layered chalk rubble taken from a surrounding ditch, and began as a 40 metre diameter mound which later increased to 160 metres. Its purpose remains unknown. The mound is part of the Avebury World Heritage Site.