An aspect of the various flood legends is the belief that whole cities have been inundated, of which the best-known Celtic examples are Ys in Finistère from Breton tradition and Cantre'r Gwaelod on Cardigan Bay in Welsh tradition. In Ireland there are thought to be cities under Lough Neagh, Lough Ree, and the Shannon River; Shannon City reappears every seven years and mortals who see it are doomed to die. Lancarrow in Cornish tradition is a city submerged beneath the sands. See also TÍR FO THUINN [Land Under Wave]. See also F. J. North, Sunken Cities (Cardiff, 1957).