The name misleads. A fjord is a drowned glacial valley; the Limfjord is a shallow sound that runs clear across the Jutland peninsula, roughly 180 kilometres from the Thyborøn Channel on the North Sea to Hals on the Kattegat. Bays open off it, narrows pinch it, and it widens again around a scatter of islands: Mors is by far the largest, with Fur, Venø, Jegindø, Egholm and Livø the memorable rest. The water is shallow almost everywhere, its deepest hole about 24 metres at Hvalpsund. Even the name is earthbound rather than maritime. It comes from an old Norse word for lime or sticky clay, the stuff once smeared onto house walls. Since Viking times Danes have thought of it as an inlet, not a sea road. For most of recorded history they were right.

There had been a way through before. Place names and the lie of the land suggest a Viking-age passage somewhat south of the present one, between Ferring Sø, which locals still call 'the Fjord', and Hygum Nor. Canute the Great sailed into it in 1027 on his way home from England. Saxo Grammaticus recorded that it had closed by around 1200, and for six centuries afterwards the sea only rehearsed: flood after flood punched briefly through the sand and the sand healed over. The 1825 breach held. It was named the Agger Channel, and it proved almost as unstable as its predecessors. In 1862 another flood tore a second opening, the Thyborøn Channel, through what remained of the Agger Tange. Agger Channel silted up and closed for good in 1877. Everything keeping the western door open since is human work: dredging, groynes, relentless sandpumping, two road dams along the inward side. A harbour went up at Thyborøn between 1914 and 1918, and a town grew around it.

The breach redrew a commercial map overnight. Western North Jutland lost its road link to the mainland, which was painful, but the harbours of the western Limfjord suddenly faced the North Sea directly, which was transformative. Aalborg, at the far eastern end, was not pleased. From the 1840s the western route mattered more and more: Britain had opened to imported grain, and ships could sail home loaded with British coal. One obstruction remained. Where the wide western basin meets the narrow eastern arm, at Løgstør, lay the Løgstør Grunde: sandbanks so shallow that big ships had to be unloaded, walked past and reloaded. The Frederik VII Canal opened in 1861 to bypass them. Forty years later the banks were simply dredged away, and the canal became obsolete more or less at once. It survives as a heritage site: a beautifully engineered answer to a problem that no longer exists.

The 1825 flood changed the water itself. What had been brackish became salt, with a steady current running west to east, and the fjord now carries oceanic salinity of roughly 30 PSU at all depths. Animals and plants from the Skagerrak and the North Sea moved in and stayed. Because the water is shallow it warms quickly from mid-April onward, and warm windless summers bring hypoxia, as does heavy winter ice. The compensation is edible and famous. Limfjord blue mussels are excellent, and its oysters are prized for a size and quality Danish cooks will argue about at length. Without the storm surge of 1825 and the ocean water it let in, none of them would grow the way they do. A 1946 act authorised closing the western channel again with dams and sluices at Thyborøn. It was never carried out; the idea resurfaced in 2005 and is still under study, because in long westerly spells the water piles up and the western harbours go under.

Before geology had an explanation, there was a pig. In the old story a woman gave birth to a hog called Limgrim, which grew until its bristles showed above the treetops, rooting channels through the ground as it wandered. One day it reached the sea, the water followed it in, and the eastern inlet at Hals was made. A medieval ballad gives it a Christian gloss: peasants haul the pig before a Thing to answer for their ruined crops, and it is sentenced to the breaking wheel. Denmark's environmental surveillance vessel Limgrim, based at Skive, carries the name still. The fjord also produced writers in quantity. Jens Peter Jacobsen came from Thisted, Johan Skjoldborg from Hannæs, Jakob Knudsen from Aggersborg, Jeppe Aakjær and Marie Bregendahl from Fjends, and the Nobel laureate Johannes Vilhelm Jensen and his sister Thit Jensen from Farsø. Thøger Larsen of Lemvig belonged to the symbolism of the 1920s, and Johannes Buchholtz kept an open house in Struer that became their meeting point. Several of those homes are museums now. And Erik Bertelsen of Harboøre wrote the line the whole region sings: Blæsten går frisk over Limfjordens vande.