Everything the Hanseatic world traded between the North Sea and the Baltic had to round Jutland and squeeze through the Øresund, where the Danish crown watched the traffic and the weather did what it liked. It was slow, it was dangerous, and it put a great deal of leverage in somebody else's hands. Look at a map of the base of the Jutland peninsula, though, and the alternative is almost taunting. The Stecknitz, a small river, flows north into the Trave and so to Lübeck and the Baltic. The Delvenau, another small river, flows south into the Elbe and so to Hamburg and the North Sea. Between their headwaters lies a low rise of no great height at all. In 1390 the city of Lübeck and Duke Eric IV of Saxe-Lauenburg agreed to cut through it.

Work began in the early 1390s and finished in 1398. The artificial cut, the Delvenaugraben, ran 11.5 kilometres; measured end to end with the rivers it joined, the route was about 97 kilometres. The trench itself was tiny by modern standards - 0.85 metres deep and 7.5 metres wide, roughly the dimensions of a suburban swimming pool stretched across a county. The summit reach stood about 17 metres above sea level, and the barges had some 13 metres of climbing and descending to do. Thirteen locks handled it at first, later seventeen. Most began as flash locks, single gates that released a surge of water to shoot a boat through, and were converted over the centuries into proper chambers. On the northern descent the Hahnenburg locks came in a staircase, one immediately below the next. The Palmschleuse at Lauenburg, a chamber lock built where a mill already stood, still survives.

The men who worked the route formed their own corporation, the Stecknitzfahrer, and their trade had rules that sound almost designed to prevent anyone getting above himself. A bargee could own one barge. Just one. The vessels ran about twelve metres by two and a half, drew forty centimetres loaded, and carried roughly seven and a half tonnes of salt apiece. The journey took at least ten days in one direction, poling and hauling through seventeen locks and a hundred kilometres of shallow water, and the salt they carried was worth more per tonne than anything else moving through northern Germany. None of them got rich on it. The guild has outlived the canal, the duchy and the Hanseatic League: it still exists in Lübeck, and it still gathers once a year for the Kringelhöge.

Lüneburg's salt was called white gold and the numbers explain why the ditch was worth digging. Traffic peaked in the fifteenth century at more than 3,000 shipments a year carrying over 30,000 tonnes. Then it ebbed. By the seventeenth century the canal was seeing 400 to 600 shipments a year, five to seven thousand tonnes; by 1789 it was down to sixty-four shipments and about 680 tonnes. Proposals for a bigger Baltic-North Sea canal circulated from the seventeenth century onward without anything being built. The old channel was enlarged in 1821-23 to 1.44 metres deep and 12 metres wide, buying it a few more decades.

The end came in the 1890s, when the Elbe-Lübeck Canal was cut - straighter, deeper, sized for steam - reusing stretches of the medieval route. That canal still carries traffic between Lauenburg and Lübeck, so in a sense the Stecknitz never really closed; it was widened out of existence. The clearest survivor is the Palmschleuse at Lauenburg, preserved as a monument, a small wooden-gated chamber that looks impossibly modest for what it represents. Five hundred years of salt passed through locks like that one. Standing beside it, the thing to hold in mind is not the scale but the idea: that in the 1390s somebody looked at two streams running in opposite directions, worked out that a boat could be lifted from one to the other, and then went and did it with spades.