Silbury Hill dates back 4,000 years old to the neolithic period

Silbury Hill, near Avebury in Wiltshire, is thought to date back 4,000 years to the neolithic period.

Archaeologists from English Heritage began investigating the hill three years ago after a hole appeared in the top, prompting fears it could collapse.

With the help of seismologists and engineers they have established the mound is safe, and now their findings could decide the future of the site.

Various shafts and tunnels have been built to explore the hill

The last phase of their work is complete and their report into the hill will help English Heritage design a long-term plan for the site.

The hill has dominated the landscape and attracted visitors for centuries, but why it was built has remained a mystery.

Various attempts have been made to understand what it is - the Duke of Northumberland sank a vertical shaft into it in 1776 and a number of tunnels have been dug since.

Theories as to its purpose abound - many believe it was a sacred monument while others think it is a Stone Age waste tip.

In recent years it has attracted its share of controversy, with alien hunters claiming it is a landing place for UFOs.

Some of them even broke in to the site, which had been cordoned off by English Heritage, abseiling down into its centre.