For centuries, historians and archaeologists have puzzled over the many mysteries of Stonehenge, the prehistoric monument that took Neolithic builders an estimated 1,500 years to erect. Located in southern England, it is comprised of roughly 100 massive upright stones placed in a circular layout. Whi1e many modern scholars now agree that Stonehenge was once a burial ground, they have yet to determine what other purposes it served and how a civilization without modern technology—or even the wheel—produced the mighty monument. Its construction is all the more baffling because, while the sandstone slabs of its outer ring hail from local quarries, scientists have traced the bluestones that make up its inner ring all the way to the Preseli Hills in Wales, some 200 miles from where Stonehenge sits on Salisbury Plain. Today, nearly 1 million people visit Stonehenge, a UNESCO World Heritage Site since 1986, every year.