Sir Winston Churchill famously remarked that the only thing that gave him serious cause for concern during World War 2 was the threat posed by Germany's submarines.
They were quick and cheap to build in large numbers and they usually sank many times their own tonnage before being sunk themselves. For an island nation dependent on maritime trade, this was bad news.
What is less well known is that U-boats existed in World War One also, and sank many ships, Royal Navy and merchant marine alike -- about five thousand of them, or twelve million tons in all.
Also less well known, at least in the Anglophone nations, is the fact that Austria-Hungary had a small submarine fleet, but the French managed to keep it bottled up in the Adriatic for the duration.