The surrounding circular earth bank and ditch, which constitute the earliest phase of the monument, have been dated to about 3100 BC. The site and its surroundings were added to the UNESCO's list of World Heritage Sites in 1986 in a co-listing with Avebury Henge. It is a national legally protected Scheduled Ancient Monument. Stonehenge is owned by the Crown and managed by English Heritage, while the surrounding land is owned by the National Trust.