The site covers 780 ha (1,927 acres) and contains around 40 passage graves, as well as other prehistoric sites and later features. The majority of the monuments are concentrated on the north side of the river. The most well-known sites within "Brú na Bóinne" are the passage graves of Newgrange, Knowth and Dowth, all known for their collections of megalithic art. Each stands on a ridge within the river bend and two of the tombs, Knowth and Newgrange, appear to contain stones re-used from an earlier monument at the site. Newgrange is the central mound of the Boyne Valley passage grave cemetery, the circular cairn in which the cruciform burial chamber is sited having a diameter of over 100 metres. Knowth and Dowth are of comparable size. There is no "in situ" evidence for earlier activity at the site, save for the spotfinds of flint tools left by Mesolithic hunters.