This article is from the Nordic countries FAQ, by Antti Lahelma and Johan Olofsson, with numerous contributions by others.
This world had for its centre a great tree, a mighty ash called
Yggdrasill. So huge was this tree that its branches stretched out over
heaven and earth alike. Three roots supported the great trunk, and one
passed into the realm of the Aesir, a second into that of the
frost-giants, and a third into the realm of the dead. Beneath the root
in giant-land was the spring of Mimir, whose waters contained wisdom
and understanding. Odin had given one of his eyes to drink a single
draught of that precious water.
Below the tree in the kingdom of the Aesir was the sacred spring of
fate, the Well of Urd. Here every day the gods assembled for their
court of law, to settle disputes and discuss common problems. All came
on horseback except Thor, who preferred to wade through the rivers
that lay in his path, and they were led by Odin on the finest of all
steeds, the eight-legged horse Sleipnir. The gods galloped over the
bridge Bifrost, a rainbow bridge that glowed with fire. They alone
might cross it, and the giants who longed to do so were held back.
Near the spring of fate dwelt three maidens called the Norns, who
ruled the destinies of men, and were called Fate (Urdr), Being
(Verdandi), and Necessity (Skuld). They watered the tree each day with
pure water and whitened it with clay from the spring, and in this way
preserved its life, while the water fell down to earth as dew.
The tree was continually threatened, even as it grew and flourished,
by the living creatures that preyed upon it. On the topmost bough sat
an eagle, with a hawk perched on its forehead: the same eagle,
perhaps, of whom it is said that the flapping of its wings caused the
winds in the world of men. At the root of the tree lay a great
serpent, with many scores of lesser snakes, and these gnawed
continually at Yggdrasill. The serpent was at war with the eagle, and
a nimble squirrel ran up and down the tree, carrying insults from one
to the other. Horned creatures, harts and goats, devoured the branches
and tender shoots of the tree, leaping at it from every side.
 
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