Ilmarinen's defining feat is not the Sampo but the sky itself: the tradition holds that he hammered out the dome of heaven, so perfectly that no hammer-mark can be seen, and his name comes from ilma, "air, sky." This marks him, in origin, as an old sky- and weather-god later drawn down into the epic as a craftsman beside Vainamoinen. His masterwork, the Sampo, is one of the great unexplained objects of world mythology: a wonder that grinds out grain, salt and money and brings ceaseless prosperity to whoever holds it, forged as a bride-price and fought over until it shatters into the sea, whence (the songs say) the sea's riches come.
Scholars have read the Sampo as a mill, a world-pillar, a heavenly vault, even a talismanic image of the cosmos; the poetry never defines it, and its very obscurity is part of why it has fascinated interpreters from the Romantics onward. The word behind his name reaches deeper than Finnish alone: cognates for ilma across the Finno-Ugric languages, including Khanty Num-Ilem, Komi Jen and Udmurt Inmar, allow linguists to reconstruct a shared proto-Finno-Ugric sky-god called something like Ilma, which would make Ilmarinen a late, localised Finnic descendant of a very old figure. His identity as a smith is thought to be a later accretion, layered on through contact with iron-working neighbours, the Baltic- or Germanic-speaking peoples whose metalworking culture reached the Finnic world and, with it, the image of a sky-god who is also a craftsman.