They operate out of hiding and lurk in the depths of the oceans: submarines. When the 'submersibles' were first used over 100 years ago, hardly anyone believed in their usefulness. That changed during the First World War. For the first time, submarines operated militarily in larger numbers, and what were reconnaissance vessels quickly became hunters that sank enemy ships. The new weapon became a symbol of treachery, and submariners glorified as heroes by propaganda.