Every year a million visitors are drawn to the Salisbury Plain in southern England to gaze upon a mysterious circle of stones.  Stonehenge may be the best-known and most mysterious relic of prehistory.  During the 20th century excavations revealed that the structure was built in stages and that it dates back some 5000 years to the late Stone Age.  The meaning of the monument however was anyone's guess - until recently.  Now investigations inside and around Stonehenge have kicked off a dramatic new era of discovery and debate.  Who built Stonehenge?  What was its purpose?  How did prehistoric people quarry transport sculpt and erect the giant stones?  A new generation of researchers is tackling these questions finding important clues in the landscape surrounding Stonehenge -one of the densest concentrations of prehistoric structures in the world.  The story of Stonehenge is being rewritten.