The Mile is not ancient. It formed during the 1500s and 1600s, when the climate cooled into the Little Ice Age and Vendsyssel's farmers, pressing harder on thin soil, stripped the coastal heath of the vegetation holding it in place. The Skagerrak did the rest. Wind lifted sand off the west-coast beaches and drove it inland in sheets, and for roughly two centuries the sand won. Some drifts reached seven kilometres from the shore. Farms went under. Whole districts were given up by people who could not dig fast enough to stay. What is now a curiosity photographed by more than a quarter of a million visitors a year was, for the families who lived alongside it, the slow confiscation of everything they owned.

Two kilometres southwest of Skagen, a whitewashed tower with a crow-stepped gable stands alone in the heather. It is all that remains above ground of the church of Saint Lawrence. Built between 1355 and 1387, it served a prosperous fishing parish until drifting sand arrived around 1600 and began, patiently, to take it. By 1775 the congregation had to dig the door free before every service. Twenty years of that was enough. In 1795 the church was closed by royal decree and the body of the building pulled down, its stone carted away and reused; the tower was left standing because sailors steered by it. About eighteen metres of it is visible today. Danes call it Den Tilsandede Kirke — the sand-buried church — and it is the plainest surviving argument for why the dunes frightened people.

Denmark's answer came late, but it was thorough. The Sand Drift Act of 1857 gave the state power to buy or expropriate land that was drifting, and crews went to work with the only two tools that actually held sand: marram grass, whose roots knit the surface together, and conifers, which broke the wind. After 1880 the plantations spread across northern Jutland in dark, regimented blocks that still define the region from the air. The dune belt was never rich — a little sheep grazing, some inshore fishing — but it stopped moving. By the 1950s the sand drift of Vendsyssel was, for all practical purposes, finished. Every significant dune in the north had been pinned down. Every dune but one.

The state bought Råbjerg Mile in 1900 and then, deliberately, did nothing to it. Formal protection followed in 1921, covering 215 hectares of sand left free to travel, and further orders came in 1962, 1964, 1976 and 2008, all of them insisting the area simply be allowed to remain in its natural state. The reasoning was as much educational as scientific: future generations ought to be able to see what their ancestors had spent three hundred years fighting. The warning has become an attraction instead. In 2018 the Mile was named to Denmark's Naturkanon, the official list of the country's natural treasures, alongside Skagen itself.

Climb to the high point, forty metres above the sea, and the mechanism is obvious. The windward slope is a long, firm, gentle ramp; the leeward face is steep, soft and collapses under your boots. Sand blows up the shallow side, spills over the crest and slides down the front, so the whole mass advances while every individual grain merely falls forward. Behind it the Mile leaves a low, wet floor of sand trailing back west toward the Skagerrak, a scar marking every metre it has covered. Ahead of it lies the main road to Skagen. At the present rate the sand will be across that road within one to two hundred years, and past the road there is not a great deal of Jutland left before the water starts again.