Few naval leaders had seen the potential of the submarine before World War I. But the British Grand Fleet was the first to feel the deadly force it could wield. On September 1, 1914, Admiral Sir John Jellicoe received word of submarine sightings near the main British naval base at Scapa Flow. His battleships were forced to flee, in humiliating fashion, to the open waters of the North Sea, where destroyer flotillas could protect them. Until antisubmarine defenses could be put into place at Scapa Flow, Jellicoe was compelled to use Lough Swilly on the Irish coast as his main base. But disaster struck soon. At the close of September, three obsolete armored cruisers, which the Admiralty leaders had carelessly placed near the Dutch coast without destroyers to shield them, fell victim to a single submarine within less than an hour. Jellicoe soon got word that 1 ,400 officers and men, most of them recently mobilized naval reservists, had perished. The danger became even more clear on October 27, when a German submarine sank the ultramodern battleship Audacious off the northern coast of Ireland as it sailed to engage in target practice. The loss was considered so catastrophic that the Royal Navy throughout the war denied that the Audacious had been more than damaged, i In response to these calamities, Jellicoe conducted all major operations with an eye to the danger from the submarine—and also from enemy minefields. He refused to send his ships deep into enemy waters, and, notably at the Battle of Jutland in May 1916, he turned away from the enemy rather than risk exposing his precious battleships to mortal danger. Thus, the nature of naval warfare changed early in the war. The ultimate weapon, reluctant as the admirals of the day were to admit it, was no longer the battleship but the tiny underwater vessel that could put a torpedo into a great vessel's hull below the waterline. To be sure, there were British submarines at work as well. They served as a scouting force for the Grand Fleet, and they found an occasional victim in the form of a war vessel, like the German light cruiser Hela, which a British submarine sank off the coast of Heligoland on September 13. But they had few German merchant ships to target outside the Baltic.