Everywhere else along this coast, people answered the sea by building a wall. The Halligen never did. These ten small marsh islands in the North Frisian Wadden Sea sit roughly a metre above mean high tide with no protective dike at all - in most cases nothing but a stone edging to keep the turf from unravelling. Instead of ringing the land, the islanders raised the ground beneath their houses. A Warft is an artificial mound of earth, built up over generations, with the farmstead planted on its crown. The old houses were timber-framed, their posts driven several metres down into the mound so a storm surge would break around them rather than carry them off, their reed-thatched roofs pulled low into a truncated hip that gives the wind nothing to grip. The ten German Halligen carry forty-three of these mounds between them. On the highest point of each, islanders dug a Fething, a shallow freshwater pond for the livestock, and caught roof rain in cisterns called Sood. Both kept people alive here for centuries. Mainland water pipes only arrived in the 1960s, and Habel, Norderoog and Südfall still have none.

The Halligen exist because North Frisia used to be somewhere else. In 1362 the second Marcellus flood tore the coastline apart and drowned an estimated ten thousand people. The Burchardi flood of 1634 finished the job, killing perhaps six thousand more and shattering the large island of Strand into fragments; Nordstrandischmoor and the Hamburger Hallig are pieces of it still floating. Roughly a hundred Halligen have vanished since - Ockholm in 1515, Dagebüll in 1703, Galmsbüll abandoned in 1806 and erased by the flood of 1825. That February surge is remembered as the Große Halligflut, the great Hallig flood: seventy-four islanders died and around two hundred and thirty lost their homes outright. Many of them gave up and moved to Föhr. The name itself carries the memory. Hallig is cognate with the Old English halh, meaning slightly raised ground isolated by marsh - high enough to stand on, and no higher.

Fewer than three hundred people live out here in total. Langeneß is the largest at about ten square kilometres, with twenty-one Warften and a little over a hundred residents; Hooge has ten mounds and a similar population. Gröde, with two mounds and eleven inhabitants, is one of the smallest municipalities in Germany. Nordstrandischmoor runs a one-room schoolhouse for eighteen people. Two narrow-gauge lorry railways cross the tidal flats on low causeways - Dagebüll to Oland and on to Langeneß, and Lüttmoorsiel out to Nordstrandischmoor - carrying groceries, schoolchildren and coastal-protection crews on open flatcars, occasionally through standing water. Norderoog and Habel are bird sanctuaries rather than villages, warden-tended. And one Hallig remains in Denmark: Langli, in Varde Kommune, a single Warft on three-quarters of a square kilometre, reachable on foot across the flats from Ho and closed to visitors outside the weeks between mid-July and mid-September.

After the catastrophic 1962 surge, Schleswig-Holstein settled on a blunt description of what the Halligen are for: bewohnte Wellenbrecher, inhabited wave-breakers, absorbing North Sea energy before it reaches the mainland dikes. The state invested accordingly - ferry berths, regular shipping, power and water, concrete safe rooms built into the old houses - and most working islanders now draw a coastal-protection wage. It worked. In 1962 the buildings were battered but nobody on the Halligen died, and when the January storm of 1976 pushed the highest water levels of the twentieth century up these flats, the Warften held. The slower threat is arithmetic. Studies on Hooge and Langeneß have measured the salt marshes gaining between 1.5 and 5 millimetres of sediment a year since 1960, against a measured sea-level rise of about 4.5 millimetres a year and projections that could exceed eighty centimetres by 2100. That is why the flooding matters. Every Landunter is a delivery of silt, and the current thinking - tested in a WWF project on climate-adapted Halligen - is to let more of it in on purpose, through controllable sluices, so the islands can keep growing at the rate the sea is rising.