Midgard Serpent - Introduction [Norse Cosmology]

The Midgard Serpent, like the Cetus, is another leviathan, but on a titanic scale. In Norse Mythology, the Midgard Serpent - "Jörmungandr" - was a massive sea snake who swam in the outer sea. His body was so large that he encircled the earth, grasping his tail in his mouth.

The Norse have a multi-tiered structure of the universe, with the Earth (or Midgard) at the center. The world tree Yggdrasil (a common motif in ancient cosmologies, especially Indo-European myths) connects the different worlds like a giant pillar, from the home of the gods (Asgard) above to Nilfeim and other lands below (home of the Trolls, the dead, etc.)

When Jörmungandr releases his tail from his mouth, it will initiate Rägnarok (the end of the world).

More info on Wikipedia:

Midgard Serpent

Norse Cosmology

Of course, you can always head to the more or less primary source, the Eddas (prose and poetic), recorded by an Icelandic monk [Snorri Sturluson], though it's been influenced by Christian thought and doesn't preserve the exact identity of the Norse myths.