“Gribshunden shows just how global medieval Denmark was during this time.”

Despite its likely origin in a Dutch shipyard, however, a new analysis has revealed surprising details about the ship’s construction. The broader switch to carvel planking happened in different ways in different regions: Dutch shipbuilders, for example, built the hull first and added the internal ribs later, whereas the Iberians constructed the frames first using specialized gauges and molds. The Iberian method—which was itself borrowed from the Italians, who learned it from the Byzantines—required sophisticated mathematical knowledge, but it was ultimately more efficient, giving ship designers greater control over the shape of the finished vessel; it was no accident that these vessels came to dominate global exploration.

This year, Rönnby and his colleague Jon Adams, a maritime archaeologist at England’s University of Southampton, examined detailed measurements of the hull’s timbers, and the early results suggest the hull was built according to the frame-first Iberian style—something no scholar expected. Castro, who was not involved in the study, says that seeing this ship design so far north at this time would be “exciting and important,” evidence of a “porous” world “where knowledge was traveling a lot faster and residing in more places than we previously thought.” And it means that “shipbuilding in the Baltic was not that far behind, if it was behind at all.” Like the famous explorers and conquerors of the Iberian peninsula, northern Europe was “ready to build ships that could carry guns and sail into the horizon.”

This shipbuilding effort underscores Hans’ ambitions as king, says Per Seesko, a researcher at the Danish National Archives. Records show that, before it sank, Hans had sent Gribshunden as far as England, to negotiate fishing rights, and possibly farther afield. When he sailed to Kalmar with Gribshunden, it was the equivalent, Foley says, of bringing “a nuclear-powered aircraft carrier”: a projection of political and military might and, Hans hoped, proof that he was Sweden’s rightful king. For audiences used to smaller, traditional longboats, the sight of it must have been jaw-dropping. And when it sank, it was more than an embarrassment, or an economic blow, or a tragedy for the lives lost on board—“it was a military setback.”

Afterward, Hans continued on to Kalmar without his flagship, but his rival, the Swedish leader Sture, was delayed, and Hans, perhaps nervous about the comparison between Sture’s military resources and his own now-depleted fleet, didn’t wait for him. He returned home without the Swedish crown. Two years later, he conquered Stockholm by force, but he soon lost the country again. He spent the rest of his reign fighting to get it back. In 1523, Sweden won outright independence from Hans’ son, Christian II.

Scholars such as Seesko and Foley like to play a parlor game about what might have happened if Gribshunden hadn’t sunk. “It was a turning point in history,” says Foley. “You might have had this Danish Nordic state emerge as a great power,” a united Scandinavia to rival England under Henry VIII. There’s no telling how the map of Europe would have come to look. Even today the European Union might be balanced by a separate northern force.

There are also hints that Hans had bigger ambitions than control of the Baltic. A 16th-century letter reveals that Hans’ father, Christian I, dispatched his own northern voyage of discovery, financed by the Portuguese, that may have followed a route past Greenland into the North Atlantic that we know the Vikings traveled centuries earlier when they temporarily settled in North America. Some historians read the evidence as showing that, 20 years before Columbus arrived in the Americas, Christian’s ship reached “cod country”: Newfoundland.

Seesko says that Hans “would have been aware” of his father’s explorations, and Foley believes that Hans may well have had ambitions to cross the Atlantic. “We have this dynamic, forward-looking, ambitious king,” he says. If Hans had conquered Sweden in 1495, perhaps he might have pushed even farther. “Hans was trying to do something new,” Foley says. “He was trying to empire-build.” Rather than being built like a ship of discovery, then, meant to project power among his rivals in the region, perhaps Hans intended for Gribshunden to be a ship of discovery itself, with a mission to reach across the northern Atlantic toward an unknown world.

IT’S ANOTHER DAY ON THE TEMPORARY ISLAND.

The cold wind has gone and the water is as calm as a mirror. It’s time for another dive, and Foley’s head is full of what else might be hidden in the sediment. King Hans’ writing desk? The earliest known gun port? Human bones, from crew or noblemen trapped on board as Gribshunden sank? The joy of this wreck is that “we never know what’s going to come up,” Foley once told me. “Every day there is something new.” He adjusts his mask and steps into the water. Bubbles rise as he descends half a millennium back in time.