The building of Stonehenge began around 3,000 bc. The main henge is constructed from giant blue stones (some more than 6 meters/20 feet high) quarried in south Wales. They were then rafted and dragged to Salisbury Plain in southern England-a distance of more than 322 kilometers (more than 200 miles). It is not known what function Stonehenge served, although the very precise alignment of the stones leaves little doubt that the builders had a good understanding of astronomy, and that the shadows cast by the stones on Midsummer's Day (21 June) were ceremonially significant.