Yggdrasil

Yggdrasil, (PROSE: The Enemy of My Enemy Is My Enemy) also known as the Great Ash Tree (PROSE: The Curse of Fenric) or Yggdras, (PROSE: Battlefield) was, according to Norse mythology, an immense ash tree which supported the world and was continually attacked by two serpents. The Seventh Doctor referenced Yggdrasil to Commander Millington. (TV: The Curse of Fenric)

The Possibility Tree was also known as the "Great Tree", and it was a tree with huge branches and roots containing all reality, (PROSE: The Hunting of the Slook) echoing the concept of Yggdras. (PROSE: Battlefield)