U-boat 
noun
A German submarine used in the First or Second World War.This approach would be similar to that adopted to fight German U-boats in the Second World War....
- The Royal Oak, a Royal Navy flagship, was sunk by a German U-boat in Scapa Flow off Orkney near the start of the second world war.
- Six months after Pearl Harbor, U-boats landed eight German marines on American shores for sabotage missions.
Origin
First World War: from German U-Boot, abbreviation of Unterseeboot 'undersea boat'.