Saga of the Northlands
Yggdrasil the World Tree, the Tree of Life, sprouted at the creation of the world to connect and support all of existence. Everything that was not present at the creation—Odin and his brothers, Muspelheim and Niflheim, and the giant Ymir—derives from the Tree, and it is destined to burn down at Ragnarok to herald the end of this creation and the beginning of the next.
The Tree itself is unimaginably vast, holding all the Nine Worlds and moving them in their proper cycles. Few creatures aside from the gods can see or interact with the Tree directly, and none can do so while within one of the Nine Worlds unless they travel dangerously close to the edge of the world.