Yggdrasill, in Norse mythology, is the cosmic tree, the tree of the world.

Its branches stretch into Asgardh above all worlds and reach to the sky while its three roots hold it upright.
The first root welcomes the world of the gods with the three Norns who guard the spring of Urdhr; the second by the Thursi of frost, the ice giants who carry with them the eye of Odin; the third is the abode of the dead, the Niflheimr. From the latter flows the fountain Hvergelmir, from which life originates, the source of all rivers.

The trunk is the Midhgardh, in which men live.

Around the tree we find in constant battle the giant serpent Nioggrh against the eagle, while in the foliage we meet the squirrel Ratatosk, the goat Heidhrun who feeds Odin's warriors, the deer ready to eat the new shoots, and at the top a golden rooster guarding the horizon.

I found out about Yggdrasill by reading 'The Mythology of Trees' by Jacques Brosse. The tree is described as being subject to cycles of life and death and endowed with the incredible gift of perpetual regeneration, thus symbolising dynamic life.

All incredibly wonderful.