Stonehenge has never been completely explained. The general consensus is that it serves some sort of astronomical function, and many theories have been proposed to show its connection with the heavens. There are also a number of mysteries involving the construction and heritage of Stonehenge, especially the transportation of the enormous stones necessary to build the monument.

When Stonehenge was first discovered by the Romans
in 60 AD, it was thought to be a place of devil-worship; a belief that
has persisted throughout the history of its exploration. The Romans
damaged it, pulling down a great many of the stones, but couldn't destroy
the whole thing with the equipment they had. Later, in 400 AD, when
the Roman Empire was more expansive, they opened Stonehenge as a tourist
attraction, and many people from the Continent came to look at the "hanging
devil-stones."
When the Romans came, they envisioned horrible sacrifices
on the Altar Stone and Slaughter Stone-- and paid little attention to the
stones themselves. In fact, there is quite a mystery involving the
origins of the various stones of Stonehenge. The Trilithons and sarsens
are all fairly local, coming from Marlborough Downs, 18 miles away over
gently sloping fields. The bluestones, however, present us with a
problem. They are believed to come from the Preseli Mountains, in
southwestern Wales, at a distance of roughly 200 miles! The Altar
Stone is believed to come from Milford Haven in Pembrokeshire, also in
southwestern Wales! How can this be possible? To drag such
heavy stones-- each bluestone is roughly 4 tons, and the Altar Stone is
significantly heavier-- across the hilly terrain for 200 miles would be
effectively impossible.
However, one theory suggests that the stones were
not
carried that great distance . . . at least not by humans. It has
been suggested that glaciers could have deposited the stones here during
the last Ice Age. And, in many ways, this is the preferred theory.
If the builders of Stonehenge were to carry stones 200 miles from the Preseli
Mountains, they would probably select the very best bluestones that were
to be found-- but the bluestones at Stonehenge display a wide mix of good,
medium, and poor quality, suggesting that the builders simply used what
they found on Salisbury Plain.
Also of note is the layout of the site; it does
not seem to be entirely an English stone circle. Almost all other
prehistoric stone arrangements in England are laid out entirely with circles,
without any horseshoe shapes. Stonehenge is unique in this respect,
and is unique in another curious respect as well: the art carved into it.
Although most people don't know this, there are many small images carved
into the faces of the standing stones. Axes, daggers, and anthropomorphic
goddess-images have all been identified. Interestingly enough, both
the art and the horseshoe-layout are characteristics of stone circles in
Brittany, but not of England. This has led some scholars to believe
that the people who built Stonehenge were either from Brittany or heavily
influenced by Brittanic styles.

Many possible astronomical alignments
can be suggested for such a large and complex structure as Stonehenge,
and the majority have been. However, instead of dwelling on the failed
suggestions, I will here focus on the proven and the controversial.
First of all, there is a great popular opinion that the Heel Stone lines
up with the rising sun on the Summer Solstice. This is not
quite true. Actually, it doesn't quite line up right-- all of the
pictures that you see with the sun balanced exactly on the tip of the Heel
Stone were produced by moving the camera around until it looked right.
The sun appears to the left of the Heel Stone by a significant margin on
the Summer Solstice. However, it is possible that there was another
stone across the axis from the Heel Stone, and together they produced a
window to see the sun through. Also, the Heel Stone has shifted over
the years, and now lies at a 27 degree angle, towards the southwest.
The Heel Stone does mark the moonrise in the middle of its 18.6-year
swing, though.
Other astronomical alignments abound in the monument;
the southeast and northwest sides of the rectangle made by the Station
Stones lines up very well with the May Day sunset, for example. Two
of the Trilithons line up with the extreme moonrise and extreme moonset
positions, while the other Trilithons mark solistical sunrise and sunset
positions.
Finally, although many New Age spiritualists think
that Stonehenge was a druidic temple, there is no factual evidence to back
them up; Stonehenge was constructed roughly 2000 years before the Druids
came about.