SM U-31 was one of the 329 submarines serving in the Imperial German Navy in World War I. U-31 was engaged in the naval warfare and took part in the First Battle of the Atlantic. U-31 sailed from Wilhelmshaven on 13 January 1915, and disappeared shortly thereafter. It was assumed she had struck a mine and sunk with all hands, somewhere in the North Sea. But six months after her supposed sinking, she drifted ashore on the British east coast. The entire crew was dead. Upon inspection, it was determined that, while lying on the sea floor, the crew had been overcome by poisonous fumes, and the sub had floated to the surface after her ballast tanks had blown independently.