The Mackerel-class submarines were a pair of experimental prototype submarines built just prior to World War II. The two submarines were similar in size and capability to the S-class submarines built at the end of World War I and had been ordered to test the feasibility of using mass production techniques to build submarines. Given a list of categories: company, educational institution, artist, athlete, office holder, mean of transportation, building, natural place, village, animal, plant, album, film or written work, what category does the paragraph belong to?
Mean Of Transportation