Yggdrasil

Yggdrasil is the great ash at the axis of the Norse cosmos, and the nine worlds hang among its branches. According to Gylfaginning it stands on three roots. One reaches toward the Aesir, above the Well of Urd, where the gods ride over the rainbow bridge each day to hold council and the three norns water the tree with well-water and white clay. One reaches toward the frost giants, above Mimir's well of wisdom, in which Odin's eye lies sunken. One reaches over Niflheim, above the roaring spring Hvergelmir, where the dragon Nidhogg gnaws the root from below.

The tree is a noisy ecosystem. A wise eagle sits in the crown with the hawk Vedrfolnir perched between its eyes; the squirrel Ratatosk runs up and down the trunk carrying insults between the eagle above and the dragon below; four stags browse its new shoots. The Grimnismal says the ash 'suffers more hardship than men know', yet it stays evergreen. The windswept tree on which Odin hanged himself for the runes is generally taken to be Yggdrasil, whose very name means 'the steed of Ygg', a grim byword for the gallows.

When the fires of Ragnarok consume the world, a human pair survives hidden in Hoddmimir's wood, generally identified with Yggdrasil itself, and repeoples the new earth.