[youtube]http://www.youtube.com/watch?v=LkVoUlfPTSc[/youtube] April 11 marks the 112th anniversary of the date the Navy bought its first fully submersible vessel, Holland IV, and the web is lighting up with birthday greetings, official and unofficial. The official video above was released by Naval Submarine Forces and features sailors from across the force. Operation Homefront, a nonprofit that provides financial help to sailors and wounded warriors, hailed the 112th birthday in a Tweet that noted the sub force’s abiding virtues: “Ever silent. Ever vigilant.” Former submariners celebrated by wearing their dolphins — as the silver or gold pin, worn by qualified submariners, is known…