Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A submarine is a watercraft capable of independent operation underwater. Though the term submarine most commonly refers to a large crewed autonomous vessel, historically or colloquially, submarine can also refer to medium-sized or smaller vessels, such as midget submarines, wet subs, remotely operated vehicles or submersible robots. Learn more about submarines, their functions and capabilities in this book.