Stonehenge may be the best-known and most mysterious relic of prehistory...

Every year, a million visitors are drawn to the Salisbury Plain, in southern England, to gaze upon a mysterious circle of stones. During the 20th century, excavations revealed that the structure was built in stages and that it dates back some 5,000 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? NOVA:THE SECRETS OF STONEHENGE attempts to answer the questions...7pm