Located about two miles (3.22 km) west of Amesbury, Wiltshire in Southern England, Stonehenge is a megalithic, or large stone, monument composed of standing stones and earthworks. One of the most famous prehistoric sites in the world, Stonehenge was added to the United Nations Educational, Scientific and Cultural Organization (UNESCO) list of World Heritage Sites in 1986. While the scientific dating of Stonehenge is complicated due to poor excavation records and natural erosion, archaeologists generally agree that the complex was built in several phases from 2950 to 1600 BCE. In the 1940s, archaeologist Richard Atkinson proposed a three-stage construction. This theory has since been accepted and published by English Heritage, the United Kingdom's advisor on the historic environment of England.
Stonehenge 1, the first stage of construction, took place from approximately 2950 to 2900 BCE. During this phase, a circular bank enclosure measuring about 360 feet (110 m) in diameter was constructed on Salisbury plain. Inside this enclosure exists a second circle of 56 pits, generally believed to have held wooden posts.
Although evidence of Stonehenge 2 is no longer visible, archaeologists believe that this second phase of construction took place between 2900 and 2400 BCE. Some post holes in the center of the original circular enclosure suggest that a timber structure was built inside the enclosure during this time. In addition, the outer ring of holes seems to have been used for cremation burials during the second building phase of Stonehenge.
During the third phase of construction, which spanned from approximately 2600 to 1600 BCE, builders seem to have abandoned timber materials in place of the large stones that are still visible on the site today. Stonehenge 3 has been broken down into several sub-phases. During the first sub-phase, two concentric crescents of holes were dug in the center of the original enclosure. These holes were fitted with 80 large bluestones.
The second sub-phase of Stonehenge 3 saw the arrival of large sarsen stones, brought to the site from a quarry on the Marlborough Downs. The following sub-phases denote periods of activity on the Stonehenge site during which the stones were rearranged in various patterns. During the final stage of construction, which took place around 1600 BCE, the bluestones were arranged in the horseshoe and circle pattern that is still visible today.
Much of the mystery that surrounds the study of Stonehenge has to do with the feats of engineering required to build the monument. Archaeologists have suggested that the stones were transported using timber and rope. Timber A-frames may also have been used to position the stones. It is estimated that the construction of the site may have involved some 242 years of man-work, while the working of the stones may have required up to 2,300 years of work.
While the significance of Stonehenge is very much under debate, most theories suggest that the site was built for ceremonial use. Archaeological evidence has indicated that the monument is astronomically aligned, placing particular significance on solstice and equinox points. There has been some speculation as to whether the monument could have been used to predict solar and lunar eclipses. In addition, many scientists believe that Stonehenge could have had some spiritual meaning and ritual uses for the prehistoric people who built it.