Stonehenge is a prehistoric monument located in Wiltshire, England composed of earthworks and large standing stones erected around 2500 BC. It consists of large stone blocks arranged in concentric circles, with an exterior circle of large stones and an interior circle of smaller bluestones. Inside is a horseshoe structure and slab known as the altar. The origin and purpose of Stonehenge remains a mystery as the culture that built it left no written records, and transporting the massive stones over 30 km remains unexplained.