This is a very poor drawing that I worked very hard on, my map of Yggdrasil and the Norse cosmology: Muspell, world of fire giants, Alfheim, world of light elves, Asgard, world of the gods, Utgard, world of the giants, Midgard, world of man, Dvergard, world of dark elves, and Niflheim, world of ice and darkness, land of the dead.
Yggdrasil is the tree of life in a constant state death and rebirth– the linchpin that holds all the worlds together.
Bifrost is a burning, rainbow bridge stretching between the upper world, the mid world, and the underworld.
Jörmungandr “huge monster” is a serpent so large that its body encircles the earth, growing deep beneath the sea.
Hræsvelg “corpse-eater” is a giant eagle that perches upon Yggdrasil’s top-most branches. This eagle causes the winds to blow when he beats his wings in flight.
Níðhöggr “malice striker” is a great wyrm squirming in the underworld, hatefully gnawing at Yggdrasil’s roots that prevent his escape into the upper worlds.
Such were the ideas and beliefs of the ancient Norse people, gleamed from various sagas written long ago during that dark age. Strange as they are, the ideas of an upper world, mid world, and an underworld ring with a familiarity that can be found within many other religions.