Britain's most famous prehistoric monument, on Salisbury Plain in southern England. It consists of two circles of large standing stones, one inside the other. The inner circle consisted of arches made by laying one stone across the tops of two others. Some of these have fallen, but some are still in position. Stonehenge was built between 3000 and 1500 BC. Nobody knows why it was built, but many people think it was to study the stars and planets or to worship the sun, because a line through its centre would point directly to the position of the rising sun on Midsummer's Day or of the setting sun in midwinter. Since the 1980s young people, including many hippies and New Age Travellers, have been going there for their own midsummer celebrations, but the police usually prevent them from getting near the stones. Stonehenge was made a World Heritage Site in 1986.