Others exist only as shadows, visible not to the human eye on the ground but to a camera lens pointed downward from an aircraft on precisely the right day. The enclosure at Grangebeg, County Tipperary, belongs firmly to this second category. It survives, if that is the right word, as a cropmark, a phenomenon where buried foundations or ditches affect the growth of grass or grain above them in subtly different ways, producing outlines that appear only in aerial photography under the right conditions of drought or low sun. On the ground, there is nothing to see at all.