Yggdrasil, the world-tree of Norse mythology, stretched from the deepest underworld of Niflheim, where the horrible dragon Nidhogg gnawed at its roots, up through the center of Midgard (the middle world of men), on through the rest of the nine worlds of Norse cosmology, to the very vault of heaven, where a watchful eagle perched upon its topmost branches. Upon the beak of the eagle sat a small sharp-eyed hawk, helping the eagle keep watch. The eagle, hawk, and the dragon at the roots can be seen in the illustration above, from a seventeenth-century Icelandic manuscript.
At the foot of the world-tree sat the three Norns, spinning out the destinies of men and gods. They also tended to the tree's many wounds with the water from a sacred pool at the base of the tree. Up and down the tree the busy squirrel Ratatosk carried the mutual insults between the eagle and the dragon.
Yggdrasil was sacred to Odin, who had hanged himself on it for nine days and nine nights, and it was there that while staring at the twigs and branches on the ground beneath he had first deciphered the runes that enabled writing and spells by gods and men.
At the battle of Ragnarok, in which the world would be destroyed and the Aesir gods devoured by their ancient enemies Fenris the Wolf and the Midgard Serpent, the dragon would finally gnaw through the roots of Yggdrasil and the entire mighty tree would crack and fall. The significance of the number of the warriors of Valhalla who sallied forth to do battle with the Wolf at Ragnarok has been discussed in a previous post.
Giorgio de Santillana and Hertha von Dechend explain in Hamlet's Mill that "world-trees" in ancient myth, whose branches are usually described as touching the sky or even piercing the sky, represent the celestial axis around which great machinery of the heavens turns.
They write: "One of the great motifs of myth is the wondrous tree so often described as reaching up to heaven. There are many of them -- the Ash Yggdrasil in the Edda, the world-darkening oak of the Kalevala, Pherecydes' world-oak draped with the starry mantle, and the Tree of Life in Eden. That tree is often cut down, too" (223).
Just as the branches of a mighty tree would all be displaced if the central trunk were moved, all the stars and colures (a term defined and explained in greater detail in the Mathisen Corollary) must move if the celestial axis is altered.
The world-tree of Norse mythology has many parallels in cultures that should have no connection with it, according to the current "isolationist" view of history. There is the sacred erica tree that houses the casket and body of Osiris in ancient Egyptian mythology, and the cedar whose top pierces the sky in the epic of Gilgamesh, but also suspiciously similar tales from the tribes of North America, the far-away Pacific island of Papua New Guinea and even the tiny atoll of Ifaluk in the Caroline Island chain. All of these "world-trees" are examined in greater detail in the Mathisen Corollary.
Perhaps the best illustrations of Yggdrasil the world-tree that I have ever seen come from Norse Gods and Giants by Ingri and Edgar Parin D'Aulaire (1967), which my parents gave to me when I was no older than six or seven. It is a wonderful volume and one I read many times growing up and whose drawings I spent many hours imitating on my own. All parents of young children should consider it for their libraries.