Nothing in this boat is held together by iron. The bottom is a single long lime log, hollowed and worked thin, doing the work a keel would later do; two more lime planks rise on each side to close the hull. Where planks overlap in a bevelled lap, they are literally sewn — stitched with cord of lime bast, spruce root or rawhide, then caulked and coated with pitch. Hazel ribs are lashed to cleats that the builders carved out of the plank faces themselves rather than fastening on separately, and ash thwarts brace the shell. Hollowed timbers form the stem and stern posts, which run out past the hull as the paired "beaks" that make this boat instantly recognisable, braced top and bottom by oak struts pegged and pitched into place. Ten thwarts cross the boat, each with room for two, putting at least twenty paddlers aboard; Danish estimates put the working load at around twenty-four men with weapons and gear. The hull weighs roughly 530 kilograms and sits high, able to cross shallows even fully crewed and loaded.

The boat did not go into the bog alone. Around and inside it lay 131 shields of Celtic type, 33 finely worked shield bosses, 138 iron spearheads and 31 more of bone or antler, eleven single-edged iron swords, and the remains of several mail coats — mail being genuinely rare in northern Europe this early. One spearhead runs to 43.5 centimetres. Mixed in with the war gear were bowls, boxes and a blacksmith's tools: the ordinary kit of men who expected to be away from home and to mend what they carried. Then there is the arithmetic. One hundred and thirty-one shields, for a boat that seats two dozen. Whatever came ashore at Als, it did not come in one hull.

Two of the swords were bent double before they went into the water. That is not battle damage; that is decommissioning, the deliberate ruining of a weapon so that nobody uses it again. Beneath the hull lay a dismembered horse, and with it a lamb, a calf and two dogs. Danish archaeology counts Hjortspring among the earliest of the great Nordic war-booty sacrifices, in which the equipment of a beaten force was given to a bog rather than shared out as plunder. Graves of the same centuries elsewhere in Denmark hold comparable offerings — grave goods, slaughtered horses, dogs and cattle, and in some cases people. The reading that follows is stark. The men who paddled these boats to Als lost. Their gear was gathered up, broken, and drowned by the people who beat them, and the peat kept every piece of it for the next twenty-three centuries.

Boats of this kind stayed in service for a long time after Hjortspring — at least into the 3rd century AD. The details drifted. Iron rivets turn up more and more; the paired beaks at bow and stern simplify into a single curved prow, cut from one timber or joined from several. What held was the essential grammar of the thing: a shell of overlapping planks built first, ribs fitted into it afterwards. Follow that thread through the finds at Halsnøy around 200 AD, Nydam in the 300s and 400s, Sutton Hoo in the 600s and Kvalsund around 690, and you arrive at the Viking ships. Run it the other way and it disappears into prehistory. The craftsmanship on display at Hjortspring is too assured to be anyone's first attempt, and the hull is a close match for the ships pecked into Scandinavian rock faces centuries earlier in the Bronze Age. This is not where the story starts. It is the earliest page that survived.

In 1991 a group on Als formed the Hjortspringbådens Laug — the Hjortspring Boat Guild — for the obvious reason: someone wanted to know what the thing actually did on water. Their full-scale reconstruction, Tilia Alsie, was launched in 1999. With experienced paddlers she makes around eight knots, and the profile that looks ceremonial in a display case turns out to be a fast, shallow-drafted raiding hull that can be driven straight onto a beach. She is kept at Lindeværftet in Holm, on Als west of Nordborg, not far from the bog that produced her original. The original itself — fragmentary, reassembled, unmistakable — is in the National Museum of Denmark in Copenhagen.