Passage tombs are a class of megalithic (mégas = great; líthos = stone) monument constructed during the Neolithic period across Atlantic and northwest Europe, broadly speaking from the mid-5th to early 3rd millennium BC. As their name suggests, passage tombs can contain large assemblages of human remains, but funerary or mortuary use seems to be only one of their functions. While the precise shape and design of passage tombs varies from region to region, they generally feature a passage connected to an internal chamber or chambers. Hundreds of these monuments dot the western fringes of Europe, with some of the very largest passage tombs clustered in ceremonial complexes such as the Bend of the Boyne in eastern Ireland. As a monument class, passage tombs represent some of the greatest achievements of human engineering and creative expression, involving the transport and erection of stone blocks weighing up to 180 tonnes, elaborate sequences of stone carving, and precise astronomical alignments. They include three World Heritage Sites (Archaeological Ensemble of the Bend of the Boyne, Ireland; Heart of Neolithic Orkney, Scotland; Antequera Dolmens, Spain) and are recognised by UNESCO as a feature of outstanding importance in European prehistory.