Historians believe Marden Henge in Wiltshire is the largest prehistoric circular monument in Britain and could be an "archaeological treasure-chest".

Very little is known about the now-collapsed 45ft high moat-surrounded mound because none of its original stones have survived.

However it is thought that it could have been similar to world-famous heritage site Stonehenge, only much larger. 

Now 80 archaeologists from around the world have descended on the site for a three-year, £1million dig in a bid to unlock its prehistoric mysteries. 

It is believed the huge monument may have been part of a network of huge stone circles in the area, which also included Stonehenge and the Avebury stone circles. 

Archaeologists' interest in the site was rekindled four years ago when experts discovered an ancient sauna at the site, which is thought to be the oldest building in Britain. 

It has been sugested sauna users might even have helped haul the huge stones upright at Stonehenge, or that the different monuments might have been used by competing communities.