The STO-class submarine, also known as the Sen Toku-class, Toku Gata-class, or Sensuikan-class (literally special class submarine), was a group of submarines used by the Imperial Japanese Navy during World War II. They were the largest used during the war, and could carry three seaplanes underwater in a large hanger. Eighteen were planned but only two were completed. These survived the war but were captured by the US and sunk as targets in 1946, without entering combat.