The idea was older than the country. In 1667, an engineer named Hendrik Stevin - son of the famous mathematician Simon Stevin - sketched the first proposal to seal off the Zuiderzee from the North Sea. It went nowhere. The Dutch needed the sea: it gave Amsterdam its harbour, fed five provinces with fish, and connected the Hanseatic ports along its shore to Europe and the Indies. But by the second half of the 19th century the equation had flipped. The North Sea Canal, opened in 1876, gave Amsterdam a faster, more direct route to the ocean. Overfishing had emptied the shallow bay. The Dutch population was booming and there was, suddenly, more demand for farmland than for fishing grounds. In 1891 a civil engineer named Cornelis Lely published the plan that would carry his name through the rest of Dutch history: dam the mouth of the Zuiderzee, drain large parts of the floor into polders, and turn what remained into a freshwater lake. Parliament said no. The fishing industry said no. The cost was unthinkable.

In January 1916, a storm tide breached dykes along the Zuiderzee and drowned villages from Marken to Schellingwoude. Two years later, the famine winter of 1918 sharpened the argument for new farmland. Lely, now Minister of Water Management, had his moment. Parliament approved the Zuiderzee Works in 1918. The actual construction did not start for another nine years. When it did, in 1927, the scale was almost theatrical. Ten thousand men. Twenty-seven large dredgers. A hundred and thirty-two barges. Eighty-eight tugs. Thirteen floating cranes. Two purpose-built construction islands - Breezanddijk and Kornwerderzand - rising from open water along the dyke's planned route. Ships dredged boulder clay, or till, from the seabed and dumped it into the open sea until the tip-line broke the surface. Once it did, basalt rocks and mats of woven willow were laid against its sides. Sand and then clay topped it off. Grass was planted on the crown. As the gap narrowed, the physicist Hendrik Antoon Lorentz calculated, day by day, how the tidal current would intensify through the shrinking opening - a problem so delicate that two complete lock complexes had to be timed to open at slack water.

The closing day arrived two years early. Engineers had feared the three deepest tidal trenches along the line - including the Vlieter, where the Batavian fleet had famously surrendered to the British in 1799 - would be the worst obstacles. All three turned out to be straightforward. On 28 May 1932, at low tide, the last bucket dropped. The Zuiderzee was gone. What replaced it was not yet a lake; it took years for rainfall and river water from the IJssel to flush out the salt. The dyke itself was not yet a road, either. The 32-kilometre causeway carrying what is now the A7 motorway opened officially on 25 September 1933, marked by a stark concrete monument designed by architect Willem Marinus Dudok at the spot where the gap had closed. Across the years of construction, an average of four to five thousand men worked the project daily - relieving real unemployment during the Great Depression. The final tally: 23 million cubic metres of sand, 13.5 million cubic metres of till.

Four years later, in December 1936, the Afsluitdijk faced its first serious storm. The water in the Wadden Sea climbed to within half a metre of the crown. The engineers had been working with too little data on wave run-up; the original height was, frankly, not high enough. After the catastrophic North Sea Flood of 1953, which killed more than 1,800 people elsewhere in the Netherlands, the Dutch began raising and reinforcing the dyke as part of routine maintenance. A major upgrade beginning in 2018 added approximately two metres to the height, designed to permit only limited wave overtopping under the worst predicted storm. The renovation also included an art project - Daan Roosegaarde's Icoon Afsluitdijk, which uses retroreflective coatings and bioluminescent algae to make the dyke glow under headlights at night. The motorway above carries roughly 20,000 vehicles a day. When the Dutch decided in 2011 to test their first 130 km/h speed limit, this is where they tested it: a road that runs through what used to be the sea.

Stand at the Stevin lock at Den Oever, named for the engineer who first imagined this dam 365 years ago, and look east. The water on your left, the Wadden Sea, is salt; it still feels the tide. The water on your right, the IJsselmeer, is fresh; rivers feed it, sluices drain it, twenty-five discharge gates in two complexes regulate how much escapes back to the ocean. New polders carved from the lake floor - the Wieringermeer, the Noordoostpolder, eastern and southern Flevoland - now hold towns and farms and motorways that did not exist in any human lifetime before about 1930. The Afsluitdijk is the wall that made all of it possible. Drive its full thirty-two kilometres and you can sense the audacity: a straight line drawn across open water by people who decided their map needed editing.