An excavation that hopes to reveal the secrets of the largest prehistoric henge enclosure in the country has begun at Marden, near Devizes.

Marden Henge, thought to date back to between 2400BC and 2000BC, is the largest but least understood ancient monument in Britain.

It measures 10.5 hectares, considerably bigger than Avebury, and was last investigated in 1969 when Professor Geoffrey Wainwright dated the henge from fragments of deer antler found in the area.

But archaeologist Jim Leary and his team from English Heritage is hoping to discover the henge's hidden secrets, including a 45ft mound that was destroyed in the 19th century and what could have been an artificial island in the River Avon used for sacrificial offerings.

Dr Leary said: "Archaeologists have largely ignored Pewsey Vale, preferring to investigate the chalk downland of Stonehenge or Avebury.

"They have done this totally erroneously and this investigation will restore the balance to some extent."

The dig is due to continue for six weeks.