During the First World War hundreds of Somerset soldiers were killed, and war memorials were put up in most of the towns and villages; only a few villages escaped casualties. There were also casualties – though much fewer – during the Second World War, who were added to the memorials. The county was a base for troops preparing for the 1944 D-Day landings, and some Somerset hospitals still date partly from that time. The Royal Ordnance Factory ROF Bridgwater was constructed early in World War II for the Ministry of Supply. It was designed as an Explosive ROF, to produce RDX, which was then a new experimental high-explosive. It obtained water supplies from two sources via the Somerset Levels: the artificial Huntspill River which was dug during the construction of the factory and also from the King's Sedgemoor Drain, which was widened at the same time. The Taunton Stop Line was set up to resist a potential German invasion, and the remains of its pill boxes can still be seen, as well as others along the coast. A decoy town was constructed on Black Down, intended to represent the blazing lights of a town which had neglected to follow the black-out regulations. Sites in the county housed Prisoner of War camps including: Norton Fitzwarren, Barwick, Brockley, Goathurst and Wells. Various airfields were built or converted from civilian use including: RNAS Charlton Horethorne (HMS Heron II), RAF Weston-super-Mare, RNAS Yeovilton (HMS Heron), Yeovil/Westland Airport, RAF Weston Zoyland, RAF Merryfield, RAF Culmhead and RAF Charmy Down.