Well-known though it may be, the story of the journey of Thor to the castle of the giant Utgarda-Loki in Jotunheim is a confused jumble of elements from Norse mythology, fun but flippant fairy tales, and the rather capricious pen of their compiler and synthesizer, the medieval Christian Icelander Snorri Sturluson.
For one thing, as even the most casual and distracted of readers will no doubt notice, Loki features in two different and contradictory roles in the tale. In one of these roles, he is Thor’s companion and is tested by the giants in the same way that Thor is. In the other role, he is the giant Utgarda-Loki. In fact, in a variant of this narrative, Utgarda-Loki appears before Thor bound in chains, just as Loki was. Surely, in the original version of this story, the giant whom Thor met in Jotunheim was none other than Loki himself.
And that’s the least of this story’s problems. In the form in which it’s been handed down to us by Snorri, there are numerous fairy tale elements that seem wholly out of place in any authentic pre-Christian Norse myth: Thor spends the night in a human farmer’s house, the gods battle characters who are straightforward allegories for abstract concepts, the beings who are typically called “giants” in English (whose Old Norse name meant “devourers”) are indeed distinguished by being comically large, the tone is one of frivolous amusement, and the story lacks any philosophical significance whatsoever. The story’s sole purpose is entertainment, not spiritual edification.
Nevertheless, given the existence of (equally spurious) alternate versions, at least some elements in this tale probably do go back to heathen times and reflect something of substantial religious import. However, in the bastardized form in which we know this story, it’s impossible to disentangle those elements from the doggerel. Their significance, therefore, can only remain unknown.
All of that is to say that this story is not a myth in any meaningful sense of the word, and is effectively worthless as a source of information about the pre-Christian Norse worldview.
But for whatever reason, people still want to read about this thing, so here goes:
The Tale of Utgarda-Loki
While Thor and Loki were traveling far from Asgard in Thor’s goat-drawn chariot, night overtook them and they were welcomed into the house of a farmer and his family.
To repay his hosts for their hospitality, Thor offered his goats for supper, knowing that he could bring them back to life afterwards and not be at any loss. After the meal, Thor laid the goats’ hides on the floor and instructed his hosts to place the bones on the hides after the meat had been gleaned from them.
The farmer had two children: a boy named Thjalfi and a daughter named Roskva. Despite the thunder god’s instructions, Thjalfi broke open one of the goats’ leg bones to suck out the marrow before placing it on the hide with the others.
When Thor awoke the following morning, he hallowed the goat hides and bones with his hammer, whereupon the goats sprang back to life. One of them, however, had a lame hind leg. Thor immediately intuited the reason for this, and was so furious at the farmer and his family that he would have slain them all on the spot had the farmer not offered him his children, Thjalfi and Roskva, to be his servants. Thor accepted, and he, Loki, and the children pressed onward on foot, leaving the handicapped goats behind.
The party’s goal was to reach Jotunheim, the land of the giants. They crossed an ocean and a thick, tangled forest. Just as night was falling, they came to a huge hall. They found no one inside, and decided to spend the night there.
They were jostled awake by a great earthquake. Running outside, they found a sleeping giant whose snores caused the earth to rumble and shake. Thor, who hated giants, clutched his hammer and resolved to smite this sure foe of his. But the giant awoke at the last second and seemed to be cheered, or at least amused, by the sight of Thor and his companions. The giant introduced himself as Skrymir (Old Norse Skrýmir, “Boaster”), but said that he already knew full well to whom he was introducing himself.
Skrymir picked up his glove, the great hall in which Thor and his company had slept during the night, and proposed that he accompany them on their journey. To this the god agreed, and off they went through forests and over hills.
At night, they took shelter beneath a venerable oak. Skrymir had been carrying all of their provisions in his bag, and when the giant fell asleep and the task of opening the bag fell to Thor, the god found himself unable to untie the giant’s knots. This so angered Thor that he struck the dozing Skrymir in the forehead, hoping to kill him. The giant awoke calmly and asked if a leaf had fallen on his head.
Later in the night, the giant’s snores grew so loud they echoed through the valleys like thunder. Thor, annoyed by his inability to sleep, and wanting to kill the giant, anyway, tried a second time to smite him by striking him in the head. But, much as before, Skrymir awakened and asked if an acorn had fallen on his head.
Just before dawn, Thor decided to try one more time to end Skrymir’s life. But the giant, awakened, asked if some birds had roosted above him and shaken some dirt from the branches onto his face.
Skrymir departed from Thor and his companions, and the company pressed onward toward a castle called Utgard (Old Norse Útgarðr – see Jotunheim and Innangard and Utangard for the significance of this name).
Around midday, the travelers reached their destination. The gate was locked and no one was there to open it, but Thor and the others found that they could fit through the very large spaces between the bars of the gate easily enough. Once inside, they found a hall where men sat eating and drinking. Amongst them was the king of this castle, the giant Utgarda-Loki (Old Norse Útgarða-Loki, “Loki of the Útgarðr“), who immediately recognized his new guests and set about taunting them for their diminutive size.
Wanting to salvage his dignity and that of his companions, Loki proudly asserted that no one else in this castle could eat food faster than he could. Utgarda-Loki challenged him to prove this boast by entering a contest with one of the men there, whose name was Logi (Old Norse Logi, “Fire”). A trough of meat was set before them, with Loki at one end and Logi at the other, and they were to see who could reach the middle first. They met in the middle at the same time, but while Loki had eaten all of the meat between the end and the middle, Logi had eaten the meat, the bones, and even the trough itself! Loki had clearly lost.
Thjalfi, who was an extremely swift runner, then offered to race anyone in the castle. Utgarda-Loki led him out to a race track and appointed one Hugi (Old Norse Hugi, “Thought”) to compete with him. By the time Hugi reached the finish line, he was so far ahead of Thjalfi that he doubled back to meet his contestant. They raced a second time, and once again Hugi beat Thjalfi by a long bow-shot. Still, they raced a third time, but Thjalfi fared even worse; he was still at the midpoint of the track by the time Hugi finished.
Thor then challenged anyone in the castle to a drinking contest, something at which he had no little skill. Utgarda-Loki had one of his servants fetch the kind of drinking horn from which Utgarda-Loki’s men were said to drink. When it was placed before Thor, Utgarda-Loki informed him that whoever could finish the horn in one drink was considered a great drinker, whoever could do it in two was considered fair, but no one in his retinue was such a poor drinker as to be unable to finish it in three.
Thor drank mightily, but by the time he had to pause for a breath, the level of liquor in the horn had barely lowered. So he gave it a second try, straining to gulp and gulp until his breath failed him. This time, the level had gone down appreciably, but the better part of the horn still remained. His third drink was even more formidable than the previous two, but in the end, much was still left. By that point, however, Thor could drink could no more, and gave up.
Then Utgarda-Loki suggested that Thor attempt to simply lift his cat from the floor, but Thor proved unable to do even this.
In a rage, Thor challenged anyone in the castle to wrestle with him. Insultingly, Utgarda-Loki appointed an old woman, Elli (Old Norse Elli, “Age”) who was one of his servants. But the great god lost even this contest.
After this, Utgarda-Loki decided that there should be no more contests, and the company spent the night there in the castle.
In the morning, they rose and prepared to leave. After Utgarda-Loki had shown them out of the castle, he confided to them what had actually transpired in their contests, saying to Thor, “Now that you have left my castle, I shall see to it that you never enter it again. The knot on my provision bag that you almost succeeded in untying had been wrought in iron. I deflected the blows you attempted to inflict on me with your hammer; instead of my face, you hit the mountainside, and carved three gaping valleys into it. Had you struck me, I would have been killed then and there.
“Loki held his own remarkably well in his eating contest, since his opponent was none other than fire itself. So it was with Thjalfi, too – he raced against thought, which nobody could ever hope to outrun. The far end of the horn from which you drank was connected to the sea, and we were actually greatly afraid that you were going to drink it all. When you cross over the sea again, you will see how much you have lowered its level. My cat was actually the Midgard serpent, whom you succeeded in raising out of the ocean and into the sky. And, finally, you wrestled against old age, and took a long, long time to fall.
“Now, for your sake and for ours, leave, and never come back.”
Thor was so angered by this humiliating trickery that he raised his hammer and prepared to slay Utgarda-Loki and smash his castle to pieces. But when he turned to do so he saw no giant and no castle – just a vast, empty plain.
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 Turville-Petre, E.O.G. 1964. Myth and Religion of the North: The Religion of Ancient Scandinavia. p. 138.
 Snorri Sturluson. The Prose Edda. Gylfaginning, chapters 44-47.