Under Læsø the groundwater is brine. It can run to more than fifteen percent salt — considerably saltier than the sea surrounding it — and on the flat southern meadows called Rønnerne, hot dry summers do half the work unassisted, evaporating shallow pools down to a heavy concentrate. From the 1330s the islanders were boiling that concentrate in hundreds of small salt kilns, and by 1481 the operation was producing something on the order of 36 tonnes three times a year. Some historians have called it Denmark's first industry, and the claim is structural rather than sentimental: organised, systematised, export-oriented, and ravenous. Boiling brine takes fuel. First it took the woods. Then it took the peat. Then it took whatever was left, and there was not much of that.

The reckoning arrived the way ecological reckonings usually do — slowly, and then all at once. With the trees gone, nothing held the island's sand, and the sandstorms began. By the late 1500s drifting sand had destroyed Hals Church outright. The crown finally banned salt-boiling in 1652, though people went on doing it illegally, in small quantities, for years afterwards. What remained was an island stripped of timber and of thatching straw, inhabited by people who still had to keep the rain out of their houses. The salt had made Læsø prosperous and then made it bare, and the second thing outlasted the first by centuries.

What Læsø did have in quantity was ålegræs — eelgrass, Zostera marina — washing ashore in long windrows after every storm. Dried, twisted into bundles and layered a metre to a metre and a half deep, it made an extraordinary roof: waterproof, deeply insulating, and free to anyone willing to gather it. The weight settles over decades, so an old tanghus roof slumps and rounds until the building looks less constructed than grown. Several hundred of them once stood on the island. Thirty-six survive. In 2015 the A.P. Møller Foundation put 19 million kroner toward saving what was left, and between 2016 and 2022 twenty-four of the houses were rebuilt — a restoration project that required not just money but the recovery of a technique that had nearly stopped being practised.

The name is far older than the salt. Læsø comes from Old Norse Hlésey, 'Hlér's island' — Hlér being another name for Ægir, the jötunn who personifies the sea and whose nine daughters are the waves. The Poetic Edda puts Thor here. In Hárbarðsljóð he recounts being set upon on Hlésey by berserk women who had bewitched every man on the island; they battered his beached ship, threatened him with iron clubs and chased off his servant Þjálfi. Read one way, those women are the waves themselves. The island also appears in Helgakviða Hundingsbana II, in Oddrúnargrátr, in Örvar-Odds saga, in two skaldic kennings and in the Prose Edda's Skáldskaparmál, and the Orkneyinga saga, written between 1214 and 1231, calls it Hleesø. For 118 square kilometres of low sand and heath, that is an unusual quantity of literature.

Together with Anholt, Læsø makes up what Danes call the desert belt — the driest corner of the country, where so little rain falls in summer that streams and ponds partly dry up. The island runs 21 kilometres long and 12 wide, low all the way, fringed by mudflats, sandbanks and the largest tidal saltmarsh in Denmark outside the Wadden Sea. Rønnerne, 1,750 hectares of it, was protected in 1980 and carries wild orchids. Harbour seals breed along the coast. The whole area has been a Ramsar site since 1977, now covering 66,548 hectares, and it matters internationally for wintering, moulting and staging waterbirds — though grazing animals have grown scarce, and scrub and invasive Japanese rose have been advancing across ground that used to be open pasture.

Since the 1990s a reconstructed medieval salt house has stood at Rønnerne, boiling brine down the way the records describe — half archaeological experiment, half tourist attraction, selling flaky sea salt that visitors carry home in paper sacks. It is a strange kind of monument: the industry that deforested the island, revived at a scale small enough to be harmless. Fewer than 1,800 people live on Læsø now, in Denmark's least populous municipality, and the figure has been falling for years. Byrum is the main town; the ferry from Frederikshavn takes about ninety minutes into Vesterø Havn, while Østerby Havn handles the fishing boats. The painter Per Kirkeby kept a house and studio here. And in 1993 the island became a legal sanctuary for the dark European honey bee, Apis mellifera mellifera, which touched off a dispute with keepers of other strains so protracted that it went through the courts and ended, years later, with the island formally divided between them.