Submarines were first widely used during World War I (1914–1918), and now figure in many navies large and small. Military uses include attacking enemy surface ships (merchant and military), attacking other submarines
usually located amidships, which houses communications and sensing devices as well as periscopes. In modern submarines, this structure is the "sail" in American usage, and "fin" in European usage. A "conning tower" was a feature of earlier
They range from small autonomous examples and one- or two-person vessels that operate for a few hours, to vessels that can remain submerged for six months