STONEHENGE, in southwestern England, means ''hanging stones''. It began as a ditch and an earthen wall that circled about the site. Experts say it probably was begun by prehistoric people around 5,000 years ago. A ring of large grey sandstones and two circles of blue stones were added over the course of hundreds of years. Even the smaller stones weighed several tons. Experts say this was done even before the major pyramids in Egypt were built. Most people believe that Stonehenge was a religious site, and that the people who built it used the cycles of the sun and moon in their worship.