Niflheim

Niflheim is one of the two primal worlds that collided with each other and created the basis for all the other worlds, fire and ice, the beginning of life. This is the realm of mists, and most of it is frozen. Most of the realm, like I have said, is frozen, but because of a great river called Elivagar, a poisonous river with a particularity which is seldom seen in our own world and even the the other world. The river turned everything into crystallized frost. Ancient mountains lie beneath the frozen wastelands, glaciers can be seen all over the place and nothing grows wherever the river and its streams spread.

The myth goes, that when Muspellheim, the fire world, collided with Niflheim, the frost began to melt away in the southern parts of the realm, and from this event the frost giant Ymir was created.
Niflheim, or most of it, to this day is still frozen and very cold, for the river Elivagar still runs in the depths of the icy realm.

Think of Niflheim as an arctic area, where the changing of seasons is seldom noticed by the people who are not used to this kind of weather, unfamiliar with a snow and ice covered world. As nowadays everybody seems to say "Winter is Coming!" because of the Game of Thrones series, well... in Niflheim Winter isn't coming, Winter is always there!
The days are long but the nights are even longer and the skies are rarely clear, so this might give the impression that it is always dark, for those who are able to travel there.

As I have written, more than half of this realm is covered in ice and snow and it doesn't seem to melt, ever. It is a land with thick fogs, blizzards, snow storms, dense mists and occasional thunderstorms. Suffice to say, this is a very dangerous realm, the nature itself will deal with any traveler.

It seems that this realm and its ice melts where it comes near to Svartalfheim, where the terrain is more mountainous. Most of this part is water formed by the melting ice, so much that ancient mountains now look like islands scattered in the water till it reaches Svartalfheim. There are a lot of icebergs floating as you might imagine.

This is the only realm with no structures whatsoever, there are no buildings, exept in the border between Niflheim and Helheim. However, there are inhabitants in this frozen realm, the famous Frost-Etins, living underground in elaborated cave tunnels.

The great well called Hvergelmir lies here in Niflheim and it isn't like the other two sacred wells. Hvergelmir is an enormous hot spring with white waters, with very turbulent waters, from which many rivers flow. This great well is the source of most rivers in the Nine worlds, unlike the regular physics of how a river flows (southwards) these rivers flow upwards, it is natural, we do have some rivers in our own world that do this, flowing through the mountainous terrain and their only escape is due north. However, here in Niflheim, it is uncertain whythe rivers flow upwards, it is an high realm indeed, the very mountains are beneath the snow and ice.
The major rivers that flow from Hvergelmir are so named: Svol, Gunnthra, Fiorm, Fimbulthul, Hrid, Sylg and Ylg, Vid, Leiptr, Sid, Vid, Sekin, Ekin, Gipul, Gopul, Gomul, Geirvimul, Thyn, Vin, Tholl, Boll, Grad, Gunnthrain, Nyt, Not, Nonn, Hronn, Vina, Veg, Svinn, Thiodnuma, Slidr and the great river Gjöll that flows into Helheim.

The watery area of Niflheim is filled with many small islands, like small mountaintops protruding up from the waters. Sometimes they are almost indistinguishable from the icebergs that also float around, sometimes the icebergs collide with them. In this area, as it is closed to Svartalfheim, osome duergar, who have wandered from their native land made their home here and the islands they inhabit actually have the names of the duergar who own them. The most famous is Lyngvi Island, which is claimed by a dwarf named Lyngvi (as it might be expected), in the middle of a small lake called Amsvartnir. This island is used by the Aesir, with Lyngvi's permission, to house the most famous current prisoner in the Nine Worlds, Fenris the Great Wolf, son of Loki. The water on the eastern side of the island is red colored.