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 Allfather Odin cast it into the sea. It grew so large that it could encircle Midgard, hence the name the Midgard (World) Serpent (Old Norse Midgarðsormr). Some versions of the legend 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 around 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 its venom. It was captured by Odin, thrown into the ocean and forced to circle Midgard (Earth). Its tail has subsequently grew into its mouth and as 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 Babylonian chaos monster Tiamat.