Some dragons guard treasure. Jörmungandr guards nothing — it simply is the boundary of the world, an enormous serpent grown so vast that it encircles all of Midgard from beneath the surrounding ocean, biting its own tail to close the circle completely.
Jörmungandr is one of three monstrous children born to Loki and the giantess Angrboða, alongside the great wolf Fenrir and the death-goddess Hel. Foreseeing the danger these three would eventually pose, Odin cast Jörmungandr into the vast ocean surrounding Midgard. Rather than dying, the serpent grew — and kept growing, until it was large enough to encircle the entire world and grasp its own tail in its mouth, earning it the epithet Miðgarðsormr, the "Midgard Serpent."
An independent ouroboros: a serpent biting its own tail to form an endless, world-encircling circle is precisely the same image found independently in the Egyptian, Greek and alchemical ouroboros tradition covered elsewhere on this site — a striking case of structurally similar symbolism emerging in cultures with no plausible direct contact, each arriving separately at the same visual language for cyclical, boundary-marking infinity.
The most celebrated Jörmungandr story, recorded in both the poem Hymiskviða and Snorri Sturluson's Prose Edda, has Thor going fishing with the giant Hymir, using an entire ox head as bait. Thor hooks Jörmungandr and hauls the enormous serpent up toward the surface, raising his hammer Mjölnir to strike — but the terrified Hymir, in a panic, cuts the fishing line before the blow can land, and the serpent sinks back down into the depths, the confrontation left unresolved.
This single dramatic scene was popular enough in the Viking Age to be carved in stone — surviving depictions include the Altuna Runestone in Sweden and the Gosforth Cross in Cumbria, England, both showing Thor with his foot braced against the boat, line taut, hauling up the serpent below — genuine physical artefacts that let modern viewers see exactly how this myth was pictured a thousand years ago.
The unfinished fight, finished at the end of the world: at Ragnarök, Jörmungandr finally rises fully from the sea, poisoning sky and water alike with its venom. Thor confronts the serpent once more and this time kills it outright — but Jörmungandr's venom is so potent that Thor manages only nine steps away from the serpent's body before collapsing dead himself. Their rivalry, interrupted at the fishing trip, resolves at the end of the world as mutual destruction rather than victory — one of Norse mythology's starkest images of a fight where killing your enemy and surviving the encounter are not the same thing.