Viking-Age Cosmography and Cosmology
Norsemen visualized the universe as nine worlds: three sets of three, on three different "levels". To the modern mind, the Norse description is not self-consistent. No matter, it didn't seem to bother the Norse mind at all.
At the top was Ásgarð, the home of the Æsir. Each of the gods and goddesses had their halls here, enclosed by the stone wall built by the giant mason. Also on this level was Vanaheim, home of the Vanir, the deities with whom the Æsir fought the first war, which ended in a draw. In addition, Alfheim was located on this level, the home of the light elves. None of the surviving literature describes the relationship between the light elves and the Æsir, except to note that they were regularly in one another's company.
The middle level contained Miðgarð, the middle world inhabited by men. It was surrounded by an ocean so vast that "to cross it would strike most men as impossible". Jörmangandr, the terrifying world serpent, lay on the floor of the ocean. Jötenheim, the land of the giants, lay outside Miðgarð to the east, in Útgarð, the outer world. To the north lived the dwarves in Nidavellir and the dark elves in Svartalfheim. Connecting Miðgarð to Ásgarð was Bifröst, the rainbow bridge guarded by Heimdall.
The lower level contained Niflheim, the world of the dead. Hel ruled here. The ninth world was Muspellheim, land of the fire giants, which can't be located precisely, except to say that it was to the south.
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.