Submarine Day
Submarine Day is held on April 11. This event in the second decade of the month April is annual. 
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Submarines have been used since the American Civil War. Submarines are part of the Navy. The first nuclear-powered submarine was the USS Nautilus.
To control its buoyancy, the submarine has ballast tanks that can be filled with water or filled with air. When the submarine is on the surface, the ballast tanks are filled with air. This makes the submarine's density less than the density of the water.
Cornelis Jacobszoon Drebbel was a Dutch engineer and inventor. He was the builder of the first operational submarine in 1620 and an innovator who contributed to the development of measurement and control systems, optics and chemistry.
Similar holidays and events, festivals and interesting facts
Golfer's Day on April 10
International Safety Pin Day on April 10
Barbershop Quartet Day on April 11 (On April 11, 1938 the Society for the Preservation and Encouragement of Barbershop Quartet Singing in America was founded in Tulsa, Oklahoma);
Eight Track Tape Day on April 11 (It's origin likely hails from the sixties and seventies);
Big Wind Day on April 12 (On April 12, 1934, the staff of the Mount Washington Observatory recorded the highest surface wind ever measured, anywhere on earth. This big wind was officially recorded at 231 miles per hour.);
Walk On Your Wild Side Day on April 12 (Thomas and Ruth Roy are credited with being the creators of this celebration);
Scrabble Day on April 13 (April 13th is the birthday of Alfred Butts, the inventor of the Scrabble game);
International Kiss Day on April 13