Stonehenge is a massive stone monument located on a chalky plain north of the modern-day city of Salisbury, England. Research shows that the site has continuously evolved over a period of about 10,000 years. The structure that we call "Stonehenge" was built between roughly 5,000 and 4,000 years ago and was one part of a larger sacred landscape that included a massive stone monument that was 15 times the size of Stonehenge.

The biggest of Stonehenge's stones, known as sarsens, are up to 30 feet (9 meters) tall and weigh 25 tons (22.6 metric tons) on average. It is widely believed that they were brought from Marlborough Downs, a distance of 20 miles (32 kilometers) 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 come from several different sites in western Wales, having been transported as far as 140 miles (225 km). It's unknown how people in antiquity moved them that far. Recent experiments show that it is possible for a one-ton stone to be moved by a dozen people on a wooden trackway, but whether this technique was actually used by the ancient builders is uncertain.

Scientists have also 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. Water transport by raft is another idea that has been proposed but researchers now question whether this method was viable.

No one knows why ancient people built Stonehenge, but it seems to have been arranged to face the midsummer sunrise and midwinter sunset. (Image credit: Pete Strasser nasa.gov)

Building Stonehenge

Stonehenge is just one part of a larger sacred landscape that contains many other stone and wooden structures as well as burials. Archaeologists have also found evidence for widespread prehistoric hunting and a roadthat may have led to Stonehenge.

From what scientists can tell, Salisbury Plain was considered to be a sacred area long before Stonehenge itself was constructed. As early as 10,500 years ago, three large pine posts, which were totem poles of sorts, were erected at the site.

Hunting played an important role in the area. Researchers have uncovered roughly 350 animal bones and 12,500 flint tools or fragments, just a mile away from Stonehenge, the finds dating from 7500 B.C. to 4700 B.C. The presence of abundant game may have led people to consider the area sacred.

Stonehenge at sunrise. (Image credit: Simon Wyatt)

Dozens of burial mounds have been discovered near Stonehenge indicating that hundreds, if not thousands, of people were buried there in ancient times. At least 17 shrines, some in the shape of a circle, have also been discovered near Stonehenge. A "House of the Dead" was recently discovered near Stonehenge that dates to 3700 B.C.-3500 B.C.

Around 5,500 years ago two earthworks known as Cursus monuments were erected at Stonehenge, the longest of which ran for 1.8 miles (3 km). By 5,300 years ago two massive eyeglass-shaped wooden palisades, which were set ablaze during ceremonies, were constructed at Avebury, near Stonehenge.

At Stonehenge, more construction occurred around 5,000 years ago with postholes indicating that either bluestones or upright timber posts were propped up on the site. Then, around 4,600 years ago, a double circle made using dozens of bluestones was created at the site.

By 4,400 years ago, Stonehenge had changed again, having a series of sarsen stones erected in the shape of a horseshoe, with every pair of these huge stones having a stone lintel connecting them. In turn, a ring of sarsens surrounded this horseshoe, their tops connecting to each other, giving the appearance of a giant interconnected stone circle surrounding the horseshoe.

By 4,300 years ago, Stonehenge had been expanded to include the addition of two bluestone rings, one inside the horseshoe and another between the horseshoe and the outer layer of interconnected sarsen stones.

Construction at Stonehenge slowed down around 4,000 years ago. As time went on the monument fell into neglect and disuse, some of its stones fell over while others were taken away. [In Photos: A Walk Through Stonehenge]

Life's Little Mysteries Hardcover Gift Book. 193 pages. Answers to Fascinating Questions About the World Around You. Buy Here (Image credit: LiveScience Store)

There is an interesting connection between the earlier Cursus monuments and the later Stonehenge. Archaeologists found that the longest Cursus monument had two pits, one on the east and one on the west. These pits, in turn, align with Stonehenge's heel stone and a processional avenue.

"Suddenly, you've got a link between [the long Cursus pit] and Stonehenge through two massive pits, which appear to be aligned on the sunrise and sunset on the mid-summer solstice," said University of Birmingham archaeologist Vincent Gaffney, who is leading a project to map Stonehenge and its environs.

Some of the people who built Stonehenge may have lived near the monument at a series of houses excavated at Durrington Walls. Recently, archaeologists discovered evidence that people who lived in these houses feasted on meat and dairy products. The rich diet of the people who may have built Stonehenge provides evidence that they were not slaves or coerced, said a team of archaeologists in an article published in 2015 in the journal Antiquity.

Why was Stonehenge constructed?

Many theories have been put forward so to why Stonehenge was constructed.

"It's part of a much more complex landscape with processional and ritual activities that go around it," Gaffney told Live Science, noting that people may have traveled considerable distances to come to Stonehenge.

One theory about Stonehenge, released in 2012 by members of the Stonehenge Riverside Project, is that Stonehenge marks the "unification of Britain," a point when people across the island worked together and used a similar style of houses, pottery and other items.

It 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.

"Stonehenge itself was a massive undertaking, requiring the labor of thousands to move stones from as far away as west Wales, shaping them and erecting them. Just the work itself, requiring everyone literally to pull together, would have been an act of unification," said professor Mike Parker Pearson of the University of Sheffield in a news release.

Stonehenge is arguably one of the most famous megalithic monuments in the world. It's also one of the most mysterious, with its prehistoric concentric rings garnering plenty of speculation as to why and how they were constructed.

Megalithic Mysteries: Test Your Stonehenge Smarts

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