excerpt from the article "Creatures in Norse Mythology" via Life in Norway
For humans at least, there are few creatures more important than Jörmungandr. Also known as the Midgard Serpent, he encircles the Earth keeping everything in place.
Jörmungandr is the child of Loki and the Jötunn Angrboða, along with Fenrir and Hel, and was tossed into the ocean by Odin. The serpent grew so large that he was able to surround Midgard and grasp his own tail.
Jörmungandr's arch-enemy is Thor, the God of Thunder. Ragnarok – the End of Days – begins when Jörmungandr releases his tail and comes out of the ocean to poison the sky.
He will be slain by Thor who then drops down dead from being poisoned by his venom. Mutually Assured Destruction!