Yggdrasil (Old Norse Yggdrasill or Askr Yggdrasils) is the mighty tree whose trunk rises at the geographical center of the Norse spiritual cosmos. The rest of that cosmos, including the Nine Worlds, is arrayed around it and held together by its branches and roots, which connect the various parts of the cosmos to one another. Because of this, the well-being of the cosmos depends on the well-being of Yggdrasil. When the tree trembles, it signals the arrival of Ragnarok, the destruction of the universe.
