Yggdrasil, the World-Tree, the tree of fate, arises in the center of the Midgard. Its branches reach up over Asgard. The entire universe is dependent on the World-Tree. The tree has three three roots. One reaches into the underworld Hel, another to the world of the Frost-Giants, and the last one to the world of human beings. Beneath the tree is the Urda well, guarded by the Norns, the three Goddesses of Fate. Two other wells also feed Yggdrasil. One is called Hvergelmer, and the other is Mimer's well. The dragon Nidhog lies in Hvergelmer and gnaws on the roots of the tree. Mimer's well is the well of wisdom, guarded by the wisest of all beings, Mimer. Odin once gave his right eye for a drink of the water from this well.