While a feet of engineering, purists point out this was not fully submerged. The sub returned to the surface regularly for navigation, poking up its periscopes for star fixes and photographs. The ability to navigate the world ocean totally free of the surface, perhaps spending months under ice, had not yet happened.

That's them picking definitions. The ship stayed submerged the whole time. They just poked their periscope out and vented for air once a day or so. There was no other way to do it at the time. They surfaced once for a medical evacuation of a sailor - they didn't have to do that so I don't think that counts.