Niflheim ("house of mists") is the far northern region of icy fogs and mists, a primordial region of cold and darkness that existed from the beginning of time. Niflheim was believed to be a frozen, fog-enshrouded wasteland located in the northern region of the primordial void or chasm known as Ginnungagap.
It is situated on the lowest level of the universe. The realm of death, Helheim is part of the vast, cold region. Niflheim lies underneath the third root of Yggdrasil, close to the spring Hvergelmir ("roaring cauldron"). Also situated on this level is Nastrond, the Shore of Corpses, where the serpent Nidhogg eats corpses and gnaws on the roots of Yggdrasil. After Ragnarok, there will be a hall here for the punishment of murderers, oath breakers, and philanderers.
The northernmost quarter of Niflheim was frozen solid. Here, glaciers and mountains of ice had solidified during ages past from an ancient river called Elivagar (Icy Waves), from which poison drops had long ago crystallized into frost. When the frost began to melt under the thawing influence of Muspelheim--the fiery southern realm of Ginnungagap--the first being, the giant Ymir, was formed.
The center of Niflheim contained the great source of all rivers, the boiling, torrential well called Hvergelmir (which means "roaring cauldron" or "bubbling cauldron"). One of the three roots of the great Yggdrasil, or World Tree, extended to Niflheim at the place where Hvergelmir bubbled up. Here also the serpent Nidhogg (Dreaded Biter) gnawed at Yggdrasil's root. Other monsters also lived or were chained up in some part of Niflheim: the monstrous wolf Fenrir was bound there with a magic rope, and the dog Garm guarded the entrance to Hel. The goddess Hel herself had been cast down by the principal god Odin to preside over the land of the dead.
According to the 'Prose (or Younger) Edda', from Hvergelmir flowed the rivers called Svol, Gunnthra, Fiorm, Fimbulthul, Hrid, Sylg and Ylg, Vid, Leiptr, Slidr (which means "fearful" and was a river made of knives rather than water) and the Giol--the river one had to cross in order to arrive at the gate of Hel.