Preparatory work for the Allied landings in Normandy on D-Day, 6 June 1944, included the creation of groundwater development potential maps for a large part of coastal northern France at a scale of 1:50 000, followed by summary maps covering coastal and inland areas eastwards into the Low Countries at a scale of 1:250 000. All these maps were compiled from sources available in the UK, almost entirely by the temporary Royal Engineers officers W.B.R. King and F.W. Shotton, under conditions of strict secrecy. Their purpose was to guide military enhancement of water supplies, especially by deployment of Royal Engineers well drilling teams, to sustain the invasion force in Normandy and its advance eastwards to Germany. A uniquely complete set of maps is preserved, together with many associated documents, in the Shotton Archive of the Lapworth Museum of Geology at Birmingham. The maps are more complex than those hastily prepared by German military geologists during the summer of 1940 to guide a cross-Channel amphibious assault in the opposite direction, into southern England. They are significant as the first British attempt to produce groundwater development potential maps for a large area at detailed (1:50 000) scale, and as the precursors of modern-day groundwater harvest potential maps.