Researchers at Bournemouth University believe they've discovered where the stones came from. They were quarried in the Presli Hills in Wales, 240 miles away from their present location.
The site of the quarry is about an acre, located at one of the highest points of Carn Menyn, a mountain in southwest Wales.
Archaeologist Timothy Darvill reports in British Archaeology:
"Within and outside the enclosure are numerous prone pillar stones with clear signs of working."A geochemical analysis of the stones at the quarry show they are identical to those at Stonehenge.
The Preseli Hills had spiritual meaning for the people living near Stonehenge, he says, which is why they took the trouble to drag the stones all the way to their temple, at about 2500 B.C.
Discovery has an interesting article on this research.