Marchinbar is all length and no width, a 210-square-kilometre splinter pointing northeast toward the open sea. Its northern tip is called Low Point. Sixteen kilometres to the south-southwest, the cliffs at Sphinx Head climb to sixty-seven metres, and two flat-topped hills behind them reach seventy-nine. A channel less than 364 metres wide separates Marchinbar from Rimbija, the most outlying of all the Wessels, while the Cumberland Strait cuts it off from Guluwuru to the south-east. The only settlement is Martjanba, a small family outstation on Jensen Bay in the north. This is Yolngu country, administered as part of the Gumurr Marthakal Ward of the East Arnhem Region, and almost everything beyond the outstation is left to the wind, the cliffs and the sea.

The coins surfaced at Djinjan creek, just south of Jensen Bay. Four were later identified as Dutch duits, dated between 1690 and the 1780s, the kind of small change that washed around the colonial trade. The other five were the shock. Their Arabic inscriptions name a ruling sultan of Kilwa, though experts still argue whether that ruler belongs to the tenth century or the fourteenth. At the time, only one comparable Kilwan coin had ever been found beyond East Africa, recovered from an excavation in Oman. For historians who suspect that seafaring peoples reached Australia's coasts long before the Dutchman Willem Janszoon's documented landfall in 1606, the Jensen Bay find is a tantalizing, unresolved clue. A similar coin, also thought to be Kilwan, turned up on Elcho Island in 2018.

Marchinbar holds another distinction, quieter but no less remarkable. Until recently it was the last place on Earth that still sheltered a wild population of the golden bandicoot, a small, russet-furred marsupial that once foraged across northern, central and western Australia, as far south as New South Wales. Feral cats, foxes and changing fire regimes erased it almost everywhere else. On this isolated island it hung on. Recognizing how precarious a single-island survivor is, conservationists mounted a salvage operation, carefully translocating Marchinbar bandicoots to neighbouring Raragala and Guluwuru to spread the risk. The Wessels, by sheer remoteness, had become an ark.

Stand on Marchinbar and the scale of its isolation settles over you. The high east coast takes the full weight of the Arafura swell, while the gentler bays on the lee side are where people have always come ashore. The same beaches that gave up the coins still gather whatever the currents carry, from driftwood and pumice to the modern plague of discarded fishing net, the ghost gear that drifts down from trawling grounds across the Arafura and Timor seas. For the Yolngu families connected to this country, none of this is exotic. The island is not a riddle to be solved but home, sea country whose rhythms of tide, weather and season they have read across countless generations, country tied into songlines and law that long predate any soldier's fishing trip. The mystery of the coins belongs to outsiders. The island itself, and the knowledge of how to live with it, belongs to them.