The axis which unites the three levels and the nine worlds is the world tree Yggdrasill. Yggdrasill is an ash tree with no known beginning and no known end; it survives Ragnarök. The tree has three roots. One is in Ásgarð, and under this root is the Well of Urð, guarded by the three Norns. Here, the gods meet in council every day. The second root is in Jötenheim, next to the spring of Mímir, whose waters are a source of wisdom. Óðin drank from the spring of Mímir at the cost of one of his eyes. The third root descends to Niflheim, next to the spring of Hvergelmir, the source of eleven rivers.
Yggdrasill sustains many animals which live in and near the tree. The serpent Nídhögg lives near the spring of Hvergelmir and gnaws at the roots of Yggdrasill. Deer and goats live in the branches and eat the tender young shoots of the tree. On the topmost branches sits an eagle with a hawk perched between its eyes. Ratatosk the squirrel runs up and down the trunk carrying insults from the serpent at the roots to the eagle at the top.
Despite the abuse Yggdrasill receives, it is sustained by the Norns. They draw water from the well and "besprinkle the ash so that its branches will not wither or decay". The tree drips dew so sweet that the bees use it for making honey. And Yggdrasill will provide shelter at Ragnarök for the only man and woman to survive the holocaust and flood.
The great world tree is eternal and everlasting.