Stonehenge is one of the most famous man-made structures on Earth, but we still know very little about it.
Stonehenge is not the biggest megalithic monument - Avebury, a few miles north, is the biggest known stone circle in the world, and is often considered a more rewarding place to visit.
But Stonehenge is unique in its complexity - no other known circle, for example, has carefully shaped and placed "trilithons" (upright posts with lintels on top). These stones were shaped with tools, also made of stone, and fitted together with great accuracy.
Excavation in modern times has revealed that Stonehenge was built over a period of about 2,000 years, during which it took various forms, starting with a simple circular bank and ditch about 100 metres in diameter, containing a circle of wooden posts.
By 1100BC, it had reached its final form, with great sandstone blocks supporting a continuous ring of lintels, as well as a horseshoe of smaller post-and-lintel structures and a circle of yet smaller upright bluestones, the whole within a bank and ditch.
Only about half of the original 162 stones remain. Many were "mined" for building stone at some time in the past. Other features, such as the ring of filled-in holes called "Aubrey holes", after their discoverer, have been found by careful archaeology. That so many of the post-and-lintel "trilithons" still stand is a tribute to the stonemasons who worked on them thousands of years ago.
* Its setting
The whole surrounding area is filled with banks, ditches, "henges" and vestigial structures dating far back into the past.
North of Stonehenge is the "Cursus" - a three-kilometre-long pair of parallel ditches. Running away from Stonehenge is a pair of low earth banks and ditches making an avenue - assumed to be a ceremonial approach to the monument.
A couple of miles to the north-west are the vestiges of Durrington Walls - a great circular earth structure - and Woodhenge. Discovered by aerial photography in 1925, this may have been a great wooden building. Avebury, and the mysterious Silbury Hill, a huge man-made mound the purpose of which is still only to be guessed at, are further north.
Andrew Lawson, director of Wessex Archaeology, says: "This is one of the most highly researched landscapes in the world but we are at the tip of an iceberg. Much of it remains buried and just occasionally we get a little keyhole insight into what is going on."
* What was it for?
The late archaeologist Jacquetta Hawkes once said every age has the Stonehenge it deserves and desires. So during the ages that religion was at its most powerful, Stonehenge was assumed to have religious significance. In the Sixties came science and the search for astronomical links.
Now, in the Nineties, we have new-age mysticism. The problem is that we are trapped in our own time, attempting to decode a distant past in terms of our own social conventions and assumptions.
There are some solar and lunar alignments which could well be intentional - our ancestors were undoubtedly closer to nature and the sun and stars than we are. So, the main axis of the structure certainly is lined up to midsummer sunrise. Andrew Lawson says: "We are completely comfortable that there is astronomical significance to the design of late Bronze Age monuments - other stone circles have alignments, too."
His own view is that the midwinter sunset - in the opposite direction from the midsummer sunrise - would have looked more dramatic. "Because if you were walking up the avenue towards Stonehenge, you would have seen the monument consume the sun as it went down."
Archaeologists, though, are wary of pushing the astronomical links too far - Andrew Lawson points out the difficulty of interpreting them too readily.
"If we knew nothing of Christian writings, and we uncovered an ordinary church, we might note that it aligned more or less on the sunrise, with a highly coloured east window enhanced by the sunlight. The window might have an image of the crucifixion. Would we say that these people sacrificed victims to the rising sun? Interpreting philosophies from artefacts is dangerous."
* The stones
The big stones are "sarsens" - hard sandstone blocks shaped from local boulders. The post and lintel structures are held together by joints derived from woodworking practice.
Stonehenge also includes a series of upright "bluestones", which are not found in the immediate area - the nearest site is in the Prescelly Mountains of Wales. We know, from recent chemical analysis of the stones, that they did indeed come from Wales, and from several sites there. We do not know how they were moved, or what the route was, although sensible guesses are possible. Why the Stonehenge builders decided to use this particular stone from that location is yet another mystery that will probably never be solved.
The stones also bear carvings. Most are graffiti, the earliest dating from the 17th century. There are, though, some shallow carvings of Bronze Age daggers, which were only spotted in 1953, one of them by a 10-year-old boy. One dagger is thought to be of Mycenaean design, and might be - more speculation here - the "signature" of an architect brought from the east for his special skills.
* The Druids
Many people profess interest in Stonehenge because of the belief in a link with the Druids. There is no evidence for this. Stonehenge was ancient when Druids were active in England, and nothing connects them. The fancy began with romantic antiquarians in the 18th century.
Modern Druids invented themselves from scratch a couple of hundred years ago. Since then they have split into various sub-groups, none of which has a continuous link with the Druids of the first century AD, about whom little is known.