Jörmungandr, the World Serpent, is a monster in Norse mythology. It is the offspring of the Aesir, Loki, and a Jötunn, and a full-sibling to Fenrir and Hel. After it's birth the God Odin cast it into the sea. It grew so large that it could encirgle Midgard, hence the name the Midgard (World) Serpent (Old Norse Midgarðsormr). Some tellings relate that the snake's growth frightened Odin, who feared it would eat Midgard, and so defeated it by putting its tail in its mouth. It is now believed to be large enough to form a full circle aroung the world, and is most typically depicted eating its tail.
In the tale of Ragnarök, the "Final Battle" of the Aesir, we are told Jörmungandr will take its tail from its mouth, emerge from the sea and poison the sky. Thor will kill the serpent, but will walk nine paces before succumbing to the venom. It was kidnapped by the god (Odin)it was then threw into the ocean and forced to circle midgard(the earth)it 's tail has subsequentualy grew into it's mouth and the tale goes when it lets go our world will end.
Jormungandr seems to fulfill a similar role to sea monsters and Sea-Gods of many creation myths, much like Leviathan and the Seven-Headed Serpent of Sumerian myth, and the Babalonian Chaos monster Tiamat.