Yggdrasil

Yggdrasil is the sacred ash tree in Norse mythology. All nine worlds exist around it. Nidhogg eats the roots and an eagle sits on top of the tree, who has a hawk called Veðrfölnir sitting on his head. A squirrel named Ratatoskr runs up and down the tree and carries messages (not always the nicest ones) from eagle to Nidhogg.