The German U-Boat U-9 sank three British ships in an hour in September 1914, which showed what this new weapon was capable of.
The submarine (Unterseeboot in German and so U-Boat for short) was not considered very useful before the war. But when the German submarine U-9 sank three old British warships in the North Sea in September 1914 Britain realised it had potential.
Germany used U-Boats throughout the war to attack British merchant shipping in the Atlantic.
How does a submarine work?
A submarine can travel either by floating on the water or travelling underwater. It "dives" by taking water into its ballast tanks, which makes it too heavy to float. It uses a set of fins, like a fish, to control how it moves through the water. When it's time to surface again the ballast tanks are filled with air kept inside the submarine. The air forces the water out and the submarine becomes light enough to float again.
Inside a German U-Boat.