At the start of the First World War, HMS "Hood" was sunk in the passage between the southern breakwaters to protect the harbour from torpedo and submarine attack. Portland Harbour was formed (1848–1905) by the construction of breakwaters, but before that the natural anchorage had hosted ships of the Royal Navy for more than 500 years. It was "the home of the Asdics," a centre for Admiralty research into asdic submarine detection and underwater weapons from 1917 to 1998; the shore base HMS Serepta was renamed HMS Osprey in 1927. During the Second World War Portland was the target of 48 air raids and a total of 532 bombs, although most warships had moved north as Portland was within enemy striking range across the Channel. Mulberry Harbour Phoenix Units can be seen at Black Barge beach, near Portland Castle. Portland was a major embarkation point for Allied forces on D-Day in 1944. Early helicopters were stationed at Portland in 1946–1948, and in 1959 a shallow tidal flat, The Mere, was infilled, and sports fields taken to form a heliport. The station was formally commissioned as HMS Osprey, which then became the largest and busiest military helicopter station in Europe. The base was gradually improved with additional landing areas and one of England's shortest runways, at .