April 11, 2021 is commemorated by the United States submarine community as Submarine Day, the anniversary of April 11, 1900 when the American Government purchased its first commissioned submarine, the USS Holland. USS Holland was the United States Navy's first commissioned submarine, named for her Irish-American inventor, John Philip Holland, although not the first submarine of the US Navy, which was the 1862 Alligator. The boat was originally laid down as Holland VI, and launched on 17 May 1897.
Holland VI eventually proved her validity and worthiness as a warship and was ultimately purchased by the U.S. government for the sum of $150,000 on 11 April 1900. She was considered to be the first truly successful craft of her type. It was decommissioned on July 17, 1905.
A submarine is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term submarine most commonly refers to a large crewed autonomous vessel. However, historically or colloquially, submarine can also refer to medium-sized or smaller vessels, remotely operated vehicles or robots. (With material from: Wikipedia)