This from wikipedia & English Heritage.
Geophysical work by English Heritage in 1997 revealed a surrounding ditch and nine concentric rings of postholes within the stone circle. More than four hundred pits, 1 metre (3ft 3in) across and at 2.5 metres (8ft 2in) intervals, stood in rings at the site. The ditch is 135 metres (443ft) in diameter and about 7 metres (23ft) wide. A 40 metres (130ft) wide entrance was visible on the north-east side. No surrounding bank has been identified although the site awaits excavation.
The geophysical work transformed the traditional view of Stanton Drew as being a surface monument and the Great Circle is now seen as being one of the largest and most impressive Neolithic monuments to have been built. Analogous with the circles of postholes at sites at Woodhenge, Durrington Walls and The Sanctuary, it is thought that the pits would have held posts which would have either been freestanding or lintelled as they could not have supported a roof at that size. The postholes in nine concentric rings held posts up to 1 metre (3.3ft) in diameter indicating the use of ancient trees which were sacred to the druids.
Nearby and to the north-east is a smaller ring of eight stones in the centre of which the geophysical work identified four further pits. A third ring of twelve stones, measuring 43 metres (141 t) wide, stands to the south-west.
A fluxgate gradiometer survey in July 2009 investigated standing stones in the garden of the Druids Arms public house known as The Cove, which showed that the stones date from nearly a thousand years before the stone circles. The conclusion from the study was that these upright stones are likely to have been the portals or façade of a chambered tomb.