So shipments from the other large supplier, Sweden, were essential for the production of tanks, guns, ships, rail cars, trucks and other implements of war. In the northern part of the Baltic Sea, the Gulf of Bothnia, lies the Swedish port of Luleå from where in the summer a quantity of ore was shipped. It was frozen in winter, so for several months each year the Swedes shipped most of their iron ore by rail through the ice-free port of Narvik, in the far north of Norway.