Stonehenge is perhaps the most famous prehistoric monument in the world. Begun as a simple earthwork enclosure, it was built in several stages, with the unique lintelled stone circle being erected in the late Neolithic period around 2500 BC. Stonehenge remained important into the early Bronze Age, when many burial mounds were built nearby. Today Stonehenge, together with Avebury and other associated sites, forms the heart of a World Heritage Site with a unique and dense concentration of outstanding prehistoric monuments.