Stonehenge is a prehistoric monument in the English county of Wiltshire, about 2 miles west of Amesbury and 8 miles north of Salisbury. One of the most famous sites in the world, Stonehenge is the remains of a ring of standing stones set within earthworks. It is in the middle of the most dense complex of Neolithic and Bronze Age monuments in England, including several hundred burial mounds. The site and its surroundings were added to the UNESCO's list of World Heritage Sites in 1986 in a co-listing with Avebury Henge.

Archaeologists believe Stonehenge was built anywhere from 3000 BC to 2000 BC. Radiocarbon dating in 2008 suggested that the first stones were raised between 2400 and 2200 BC, while another theory suggests that bluestones may have been raised at the site as early as 3000 BC. The surrounding circular earth bank and ditch, which constitute the earliest phase of the monument, have been dated to about 3100 BC. The biggest of its stones, known as sarsen stones, are up to 30 feet tall and weigh 25 tons on average. It is widely believed that they were brought from Marlborough Downs, a distance of 20 miles to the north. Smaller stones, referred to as ‘bluestones’ (they have a bluish tinge when wet or freshly broken), weigh up to 4 tons and most of them appear to have come from the Preseli Hills in western Wales, a distance of 156 miles. It’s unknown how people in antiquity moved them that far but water transport was likely used for part of the journey. Recently, scientists have raised the possibility that during the last ice age glaciers carried these bluestones closer to the Stonehenge area and the monument’s makers didn’t have to move them all the way from Wales.

There are numerous theories as to why Stonehenge was built. At the time it was made, people in the area were herders and farmers and left no written records behind. An avenue connecting Stonehenge with the River Aven is aligned with the solstice. In addition, research at the nearby ancient settlement of Durrington Walls, a site that also contains a series of wooden pillars, shows that pigs were slaughtered at the site in December and January, suggesting that the winter solstice was marked at Stonehenge. The burials at Stonehenge offer another clue. Recent research indicates that the burials took place from its beginning, around 5,000 years ago, to its high point when the sarsen stones were set down. Among the burial goods is a mace head, an item historically associated with elite members of society. This discovery raises the question whether the people buried at the monument were local leaders and Stonehenge, in some way, commemorated them.

One new theory about Stonehenge, released recently by members of the Stonehenge Riverside Project, is that Stonehenge marks the ‘unification of Britain’ at a point when people across the island worked together and used a similar style of houses, pottery, and other items. This would explain why they were able to bring bluestones all the way from West Wales and how the labor and resources for the construction were marshaled. In a news release, professor Mike Parker Pearson of the University of Sheffield said, "This was very different to the regionalism of previous centuries. Stonehenge itself was a massive undertaking, requiring the labor of thousands to move stones from as far away as West Wales and shaping them and erecting them. Just the work itself, requiring everyone to literally pull together, would have been an act of unification.".