Stonehenge is a prehistoric World Heritage Site in Wiltshire, England consisting of a stone circle built in three phases between 3100 BC and 1950 BC. The first phase included a timber circle around 3000 BC, later replaced with stones around 2600 BC. Around 2400 BC, 30 huge sarsen stones were erected forming an inner and outer stone circle, with the remaining bluestones placed innermost. Though its purpose remains unknown, it is believed Stonehenge functioned as a ritual site and calendar based on solar alignments during solstices.