The world tree in Norse Mythology is called Yggdrasil, which is another word for Odin's Steed. It may be so named due to the time Odin spent 9 days and nights hanging from the tree in his search for wisdom. It is said the nine worlds are found in its branches and roots.
The world tree has four harts, or stags, that climb among its branches nibbling on its leaves. It is topped by an eagle with a hawk standing on its head. The serpent creature called Nidhogg nibbles on its roots. At the base of the tree is Urd's Well, or The Well of Wyrd, where there can be found three norns who care for the tree with the waters of the well. They also find time to determine the fates of all humankind. A squirrel named Ratatosk runs up and down the tree conveying insults from the eagle to Nidhogg and Nidhogg to the eagle.