Incidentally, submarines have been traversing the north pole underneath the ice (and often breaching the ice) since the Nautilus did it in 1958. I personally know plenty of submariners who have done it: https://en.wikipedia.org/wiki/USS_Nautilus_(SSN-571) And at sea, we can all see the curvature of the planet with the naked eye. A distant ship on the horizon appears only as its mast. The hull and deck are hidden behind the curvature of the earth. I've seen it plenty of times through a periscope. People who haven't spent much time staring at the open ocean looking for distant ships to run away from might not have noticed. But you can see it in photos online.