Föhr was not always an island. Until the Grote Mandrenke, the great drowning of men, tore this coast apart in 1362, it was a piece of the North Frisian mainland connected by deep tidal creeks. What the flood left is twelve kilometres long and under seven wide: marshland in the north, higher sandy geest in the south, topping out at a modest thirteen metres between Nieblum and Midlum. Sylt and Amrum stand between Föhr and the open North Sea and take the worst of the weather, which is why the island is known as the green one and why the tourist office has been selling it since 2010 as the Frisian Caribbean - a phrase that sounds absurd until you walk the fifteen kilometres of sand along the southern shore on a July afternoon. Around eight and a half thousand people live here across one town and sixteen villages. Nearly all the place names end in -um, which means home.

The whaling began with a schoolteacher of sorts. In the seventeenth century a pastor named Richardus Petri opened a private navigation school at Süderende, the first on Föhr, and made ordinary islanders into navigators - though he never went to sea himself, and Jens Jacob Eschels, who grew up here in the 1760s, recalled that plenty of young sailors thought learning navigation smacked of putting on airs. They came round. Around 1700, Föhr had roughly six thousand inhabitants and sixteen hundred whalers. At the peak of Dutch whaling in 1762, eleven hundred and eighty-six men from this one island were serving on Dutch vessels off Greenland and Svalbard, and a quarter of every shipmaster on the Dutch whaling fleet was a Föhr man. By the late eighteenth century the island held a thousand sailors, a hundred and fifty of them commanders, and their houses - low, brick, extravagantly carved - still stand in Nieblum and Süderende.

The whales ran out, and then the farms proved too small. Through the middle of the nineteenth century, families who could not make a living from a Föhr smallholding did what islanders do and left. Unemployment pushed them; so did the Prussian military service imposed after the Second Schleswig War took Föhr from Denmark in 1864. They went to North America, and - unusually for emigrants from a fishing coast - they clustered into two trades. In California, Föhr families became chicken farmers. In New York City they went into the delicatessen business, and they went into it so thoroughly that the trade became the island's second inheritance after whaling. A generation of Föhr children grew up expecting that adulthood meant either the North Sea or a shop counter four thousand miles west of it.

On 21 February the island lights the Biikebrennen, a bonfire visible for miles across the flats. On 21 December comes Tamsen, when young people hide anything that spins; at New Year, costumed groups go house to house daring neighbours to guess who they are, paid in sweets or schnapps according to age. Christmas here means the kenkenbuum, a wooden frame hung with greenery and dough figures rather than a tree. The other Föhr - the visiting one - began in 1842, when Christian VIII of Denmark took the island as his summer residence and made it fashionable. Wyk's physician Carl Haeberlin turned that fashion into medicine at the start of the twentieth century, pioneering climatotherapy and thalassotherapy in Germany on the strength of this particular air and this particular seawater. His Frisian museum, entered between two whale jawbones, opened in 1908. A century later, Alkersum got the Museum Kunst der Westküste, where Nolde, Munch and Liebermann hang in a village of a few hundred people.

Föhr collects oddities the way it collects tourists. Six duck decoys survive in the northern marshes, artificial ponds first dug in the eighteenth century as a winter hobby for sea captains with nothing to command; more than three million ducks have been taken from the pond at Oevenum since 1735, and between 1885 and 1931 a Wyk factory canned the meat for export worldwide. Five windmills stand here, one of them a buck mill carried over from Hallig Langeneß, and the Viking-era ring wall of the Lembecksburg sits in the fields near Borgsum. The island also still speaks for itself: Fering, the local variety of North Frisian, is common in the western villages, split into Westerland and Osterland forms, and it has produced its own literature - the poet Stine Andresen of Wyk, and Ellin Nickelsen, whose novella Jonk Bradlep won the first North Frisian literature competition in 1991. There is even an anthem, Leew Eilun Feer: beloved isle of Föhr.