The cosmology of Norse mythology centered primarily on nine worlds (Old Norse: Níu Heimar), divided into three levelbit s, all located around a cosmological ash tree known as Yggdrasil.
The ash tree Yggdrasil, the World Tree, was the axis of the universe. Nothing is known of its origins. Yggdrasil was populated by various specific creatures; an omniscient eagle sat at its zenith, with the hawk Vedrfolnir between its eyes. This unnamed eagle was responsible for providing air and wind to the nine worlds, by flapping its wings. A vile wyrm (or dragon), named Nidhogg, lay at Niflheim; aided by its army of unnamed serpents, it slowly gnawed away at Yggdrasil's roots. Ratatosk, a squirrel, ferried insults between the eagle and the dragon. Four stags—Dainn, Dvalinn, Duneyrr, and Durathorr—lived in the branches.
Three roots of Yggdrasil drank the waters of the worlds; one, in Asgard, tapped the well of Urd; two others were in Helheim and Jotunheim. The Jotunheim branched tapped the springs of Mimir, whose waters were said to bring rumour and understanding.
The tree was a source of life to the creatures living in it; Nidhogg and his entourage used to nibble on the roots; the stags ate its leaves and shoots; and Ratatosk continuously bit it and shed the bark away. To counteract this, the Norns that lived by the Well of Urd took a mixture water and mud and poured it over the branches everyday; the holy mixture protected the tree from decaying.