Stonehenge was built between 3100 and 1550 BC about 8 miles (13 km) north of Salisbury, England. It is a circular group of huge, erect stones. Stonehenge is probably the most important prehistoric monument in the whole of Britain and has attracted visitors from very early times. There were three main periods of building: the first period beginning around 3100 BC included the digging of a circular ditch and a ring of 56 pits, known as Aubrey Holes. In the second period, around 2100 BC, huge pillars of rocks were brought from Southwestern Wales and erected into concentric circles around the centre of the site. The double circle was never completed and was dismantled during the following period. The monument was remodelled in the third person. A circle of 30 upright stones weighing up to 50 tons each was erected and capped by ring of stone lintels. These enclosed a horseshoe-shaped formation of five pairs of upright stones, each pair capped with a stone lintel. This final phase of building probably ended before 1500 BC. Scientists believe that early people were able to foretell eclipses of the sun and the moon by the positions of these celestial bodies in relation to the stone monument. The site may have served as an observatory where early rituals or religions ceremonies took place on specific days on the year. 