 The exception to this were the new Type XXI boats, which continued to train in the Baltic Sea.To prevent this, the Royal Air Force dropped thousands of sea mines into German territorial waters, in the hope that submarines entering or leaving harbour or training in shallow waters would be lost on them.This is what destroyed "U-3519" on 2 March 1945, when she ran afoul of an air-dropped mine near Warnemünde, in position and sank to the bottom taking all 65 of her crew with her.