Nuclear powered. i.e. No need to resurface for air (as all oxygen can be recycled or separated from water) and no need to refuel for the entire lifespan of the submarine.

How long can they actually stay below the surface in practice though? I'm guessing water is filtered on the vessel, but food must be re-supplied constantly I'm again guessing? Maybe they do it like airplanes refuel in the air, rendevouz with another submarine and somehow transfer the resources for food making between then? Probably read too much Tom Clancy...