|Jörmungandr with Hel and Fenrir|
In the 13th century Prose Edda, it is said that Odin took Jörmungandr and threw him into the sea. There, the serpent grew so long that he could circle the entire earth and grasp his tail with his mouth. This is the source of the "World Serpent" name, since he literally can circle the globe.
Jörmungandr is an enemy to Thor, and it is said that the two will battle during Ragnarok. During the battle, Thor will kill the serpent, but then will himself collapse, poisoned by Jörmungandr.
There are other stories of the rivalry between Thor and Jörmungandr, including the tale of an unsuccessful fishing trip. Thor and the giant Hymir go fishing, and despite Hymirs warning, Thor wants to go very far out to sea. Once there, he takes the head of an ox and puts it on a line-- Jörmungandr bites. The two fight, but Hymir is so scared that he cuts the line and releases the serpent.