The Hørdum stone is a picture stone rather than a runestone - a slab carved with an image and, as far as anyone can tell, never with an inscription. It is gneiss, roughly 125 centimetres tall, about 50 centimetres across at its widest, and around 40 centimetres thick. Solid, but not monumental. Two people could shift it, which is precisely why somebody eventually did.

That reuse was normal. Before anyone thought of these things as heritage, runestones and picture stones were exactly what a builder wanted: flat, hard, already quarried. They went into roads, bridges, walls, and church foundations across Scandinavia. The Hørdum stone survived because it was useful, and it survived intact because the carved face happened to be laid downward, protected from four hundred years of boots and weather.

The scene comes from Hymiskviða in the Poetic Edda, and it is the closest thing Norse myth has to a fish story. Thor goes out in a boat with the jötunn Hymir, baits his hook with the head of an ox, and drops the line for the largest thing in the sea: Jörmungandr, the serpent that circles the world of humans with its tail in its mouth.

He catches it. That is the point of the story - the god of thunder, hauling the world-serpent up toward the surface with a hand-line. Snorri Sturluson's Prose Edda adds the detail the carver clearly cared about: Thor braced so hard against the strain that his feet went straight through the bottom of the boat. On the stone, that is the figure with a leg through the hull. It is a small piece of comedy and enormous strength in the same gesture, cut into gneiss by somebody who had heard the story told well.

It is worth being precise about what the stone actually shows. A boat, two figures, a line going down, one figure with a foot through the hull, and the second figure holding a tool - those are carved, and visible. The serpent is not. It has been suggested that the head of Jörmungandr can be read in the natural fracture edges of the stone below the boat, which may be a genuine intention of the carver, or may be something modern eyes have found because they knew what to look for.

The tool matters more. In the Poetic Edda's version, the serpent breaks free on its own. In Snorri's Gylfaginning, it is Hymir who panics and cuts the line, robbing Thor of the catch. A giant in the boat holding a blade, poised, points squarely at Snorri's telling - which means this stone is carrying a specific variant of the myth, in Jutland, in the Viking Age, centuries before anyone in Iceland wrote it down.

Thor and the serpent appear to have been one of the most popular subjects in Norse art. Three other monuments have been linked to the same episode: the Ardre VIII image stone from Gotland, the Altuna runestone in Uppland, and the Gosforth Cross in Cumbria - the last of these a Christian cross carrying pagan mythology, carved in England by people evidently comfortable with both.

Boats themselves are everywhere in the corpus. Runic inscriptions from Hjermind, Spentrup, Sønder Kirkeby, Tullstorp, Holmby, Farsø, Ledberg, Skarpåker, Gamla Uppsala and a couple of dozen more show ships without showing this myth. What distinguishes Hørdum is the moment chosen: not a voyage, not a departure, but a specific second of a specific story, with the line still tight.

The stone is on display inside Hørdum church, in the porch, a few steps from where it was found. There is no visitor centre and no ticket. It is a village church in Thisted Municipality, out in Thy, with the North Sea coast a short drive west and not much traffic in between.

That is arguably the right place for it. The stone was carved by people who lived on this ground, buried by people who lived on this ground, and dug up by people who lived on this ground. It has never been anywhere else. A thousand years on, the god is still pulling, the giant is still reaching for the line, and nobody has yet found out how the fight would have ended.