Svartalfheim, also called Nidavellir ("the dark fields" or "low fields"), is the subterranean realm of the dwarves in Norse cosmology, a world of caverns, tunnels, and forges beneath the earth. The dwarves who dwell there are the supreme craftsmen of the Norse cosmos, and the realm is defined by their work: nearly every one of the gods' greatest treasures was made in these underground smithies.
The Eddas credit the dwarves with forging Mjollnir, Thor's hammer; Gungnir, Odin's unerring spear; the golden ship Skidbladnir; the boar with golden bristles; the ring Draupnir that drips new rings; and the golden hair made for Sif, most of them extracted, in Snorri's telling, through Loki's scheming and the dwarves' rivalry to outdo one another. (The sources are not perfectly tidy: they sometimes conflate the dwarves with the "dark elves," and the exact relation of Svartalfheim to the other worlds is left vague.) The realm embodies a recurring theme in the mythology: that the gods depend, for the very instruments of their power, on the hidden skill of beings who live below and apart from them, that the treasures of the bright upper world are made in the dark.