Two ways across exist, and both belong to the sea. The Laaningsvejen, the low-tide road, opened in 1978 and runs from the mainland at Vester Vedsted straight over the exposed seabed, needing regular loads of fresh gravel to stay where it is. The alternative is the Mandobussen, a bus dragged by a tractor on enormous tyres, which leaves from the Wadden Sea Centre and carries something like 35,000 people a year across firm mudflat at the lowest water. Neither runs on a timetable that suits visitors. Both run on the tide, and getting it wrong is not a joke: on 18 June 2010 a tractor-drawn tourist vehicle dropped into a metre-wide hole in the seabed between Mandø and Koresand and overturned. Three passengers were killed and thirty-six were injured. The Wadden Sea is shallow, gentle-looking, and entirely indifferent.

Mandø first appears in writing in 1231, though people were living here earlier - the remains of Gammel Mandø, the old village, lie up near the mainland end of the causeway. In 1558 a storm surge tore the island in two at a channel called Store Lo and destroyed the northern settlement outright. The survivors rebuilt on the southern half, on dunes to the west, and centuries of silt and dyke-building eventually stitched the two pieces back together. The floods kept coming. A post on the island records the bad years: 1909, 1911, 1923, 1928, 1936, 1968, 1976, 1981, 1999. In 1909 the water came within five centimetres of the top of the seawall. The village dyke, Bydiget, went up in 1887 east of the settlement; its predecessor had been built largely by the island's women, because the men were away at sea. A second dyke, Havnediget, followed in 1937 to shelter Gammel Mandø. Both were breached in 1981.

For most of Mandø's history the division of labour ran along the shoreline. The men fished and later sailed; from the middle of the eighteenth century to around 1900 they shipped out as sailors, helmsmen and captains on vessels all over the world. The women ran the farms. Until the eighteenth century none of the island's fields were even enclosed, and an early autumn surge could carry off an entire hay harvest in a night. About 130 people lived here in 1652, too many to feed from the fields alone, which is what pushed the island toward fishing in the first place. The population peaked at 262 in the census of 1890, then drained away decade by decade to roughly thirty today. The church, built in 1639 and restored in 1727, sits on a raised earth mound so the congregation stays dry. The Dutch windmill dates from 1832 and, after a restoration finished in 2008, can still grind grain.

There is nearly as much mudflat on Mandø as there is arable land, and the mud is the point. Tidal marsh, littoral zone and upland grassland ring the fields, and the whole system feeds birds on an industrial scale - terns, sandpipers, waders, eiders, with roughly a thousand pairs of oystercatchers and eighty-six pairs of bar-tailed godwits counted breeding in 2011. Migrating flocks stop here because the Wadden Sea is one of the great refuelling stations of the East Atlantic flyway. Sheep graze the upland and stretches of the causeway. Overhead there is very little else: in 2024 Mandø became Denmark's third International Dark Sky Place, which on an island with thirty residents and no through traffic is less an achievement than a description.

Mandø has produced two people whose names travelled much further than the island did. Eugenius Warming, born here in 1841, spent only his first three years on Mandø before going on to become one of the founders of ecology as a science - an odd and fitting origin for a man who spent his life explaining how plants adapt to hostile ground. Villum Kann Rasmussen, born in 1909, left for school in Sorø at eighteen and went on to found VELUX; the foundations he later established have funded scholarships for Mandø's own inhabitants. The island keeps drawing outsiders trying to explain it. The 2023 documentary As the Tide Comes In followed the last twenty-seven islanders through an approaching storm surge, holding the place up as both a genuine one-off and a small, stubborn stand-in for everyone's argument with the water. Up to 80,000 day visitors come each summer to a place with one shop.