Yggdrasil

Yggdrasil was, in the Norse Mythology, the "World Tree," a gigantic ash, thought to hold all the different worlds: Asgard, Midgard, Utgard and Hel. The name literally means the 'horse of Yggr', i.e. the horse of Odin, since Yggr is one of Odin's many names.

Three roots supported the trunk, with one passing into Asgard, one into Midgard and one into Hel. Beneath the Asgard root lay the sacred Well of Urd, and beneath the Midgard root lay the spring or well of Mimir?.

The messenger in the tree (and thus between the worlds) was the squirrel, Ratatosk. In the top of the tree was perched a giant eagle? (with a hawk upon its forehead) who blew the winds over the worlds with his mighty wings.