Excavation work is being carried out at the Silbury Hill in Wiltshire, which is in need of stabilising.
Nobody knows what Silbury Hill was built for
Archaeologists will also be having another attempt at unravelling the mysteries behind the giant mound of earth.
What is it?
Silbury Hill is a 130ft Neolithic mound near Avebury. The largest man-made mound in Europe, it is comparable in height and volume to some of the Egyptian pyramids.
What is it for?
The exact purpose of the mound has not been determined. English Heritage believes there was a Roman community at Silbury Hill about 2,000 years ago and thinks the site may have been a sacred place of pilgrimage.
How did it get there?
No-one knows exactly. It is estimated that work began about 5,000 years ago and was probably completed by about 2350 BC.
But the exact origins and purpose of the mound remains a mystery.
Have there been previous excavations?
Yes - the first recorded archaeological investigation of Silbury hill began on Thursday 31 October, 1776.
Further tunnels were dug in 1849 and 1968.
Why is excavation work needed?
The tunnels created in the previous excavations were not fully filled up when they were abandoned, which means there is the threat of subsidence.
In June 2000, a large hole suddenly appeared on the top of Silbury Hill, as a result of the head of an old excavation shaft, dug in 1776, beginning to collapse.
What is the excavation work being carried out?
English Heritage has decided to remove the current inadequate backfilling to the tunnels and properly backfill them.
Also, temporary capping to the top of the collapsing shaft is to be replaced with chalk, and a monitoring programme put in place.
Can you climb up it?
Sadly not. There is no public access to the mound itself at any time.