Erected between 3000 and 1600 BC, Stonehenge is a giant stone circle that stands on Salisbury Plain and is considered today to be one of the most famous surviving sites from the ancient world. The stones, which are up to 6.7m (22ft) high, attract hundreds of visitors every day, who come to marvel at the ingenuity of those who engineered the construction of the site and the techniques used to move and position the stones. The first prehistoric structures appeared at the site in approximately 3000 BC. However, it was not until 2550 BC that the 3.9-ton (4-tonne) bluestones were brought to Stonehenge from the Preseli Mountains in Pembrokeshire, southwest Wales, using manpower alone. The 30 sarsen stones, which form the Sarsen Circle with raised lintels around the edge, were brought 150 years later from the Marlborough Downs, 32km (20 miles) away, the largest weighing an astonishing 49.2 tons (50 tonnes). Avebury, which is located 37km (24 miles) from Stonehenge, is another ancient mystery to modern-day man, and one of the biggest stone circles ever built. There are also many burial mounds nearby, as well as long barrows from the Neolithic Age, the most famous of which is West Kennet Long Barrow