Sweden has some nice AIP (Air Independent Propulsion) subs: https://en.wikipedia.org/wiki/Gotland-class_submarine https://en.wikipedia.org/wiki/Gotland-class_s

Sweden has some nice AIP (Air Independent Propulsion) subs: https://en.wikipedia.org/wiki/Gotland-class_submarine https://en.wikipedia.org/wiki/Gotland-class_submarine They are rather unique, very silent and can stay under for weeks, depending how much liquid oxygen they can carry.