The British navy dominates the North Sea. The German fleet is largely confined to port, knowing that it will be annihilated if it encounters the British in strength. But the British are still worried about raids against their commercial shipping. To guard against this, they maintain patrols of light cruisers in the North Sea.
But the German navy does not just comprise surface ships. They also have a number of submarines, known as U-boats (from the German Unterseeboot). Initially at least neither side takes the U-boats too seriously. That changes today. Commanding the U-9, Otto Weddigen encounters three British cruisers. Over the course of an hour, the submerged U-boat fires fires its torpedoes at the cruisers, sinking all three of them. The Germans suffer no losses. 1,459 British sailors lose their lives.
image source (Wikipedia)