Niflheim - also called Niflheimr, Neiflheim, Niflheimur, Nibelheim or Nebelheim ("world of mist" or "world of darkness") - is an icy world in Norse mythology. It is located in the north, under the third root of Yggdrasil.
Niflheim is not named in any of the ancient mythological poems. It is however mentioned several times in the Edda (Gylfaginning) by Snorri Sturluson, who may have invented the name.
Niflheim existed long before the creation of the world. At its center is the spring Hvergelmir, from which the rivers Élivágar flow. The meeting of the cold of Niflheim and the heat of Muspellheim above Ginnungagap is at the origin of the appearance of the giant Ymir and the birth of the world.
Still according to Snorri, Niflheim is also the equivalent of Helheim, where the goddess Hel reigns. Odin would have indeed precipitated her there, giving her the power to rule over those who die of disease or old age. She owns the residence named Éljúdnir.
Niflheim is sometimes confused with Niflhel. Thus, when they speak of how Thor killed the giant master builder of Asgard (Gylfaginning, 42), two of the four main manuscripts of Snorri's Edda say that he sent him "below Niflheim", the other two "below Niflhel".