Archaeologists have begun a major dig to unearth the hidden mysteries of a buried ancient stone circle site that is ten times bigger than Stonehenge. The enormous 4,000 year old Marden Henge, in Wiltshire, is Britain's largest prehistoric structure stretching for 10.5 hectares, the equivalent of 10 football pitches. English Heritage is carrying out a six-week dig hoping to reveal the secrets behind the giant henge which has baffled historians for centuries.