Shipwrecks from this period are exceedingly rare. Unless a ship is buried quickly by sediment, the wood is eaten away over the centuries by shipworm, actually a type of saltwater clam. But these organisms don’t survive in the fresher waters of the Baltic, and archaeologists believe that much of Hans’ vessel and its contents are preserved. That promises them an unprecedented look at the life of a medieval king who was said to travel with an abundance of royal possessions, not only food and clothing but weapons, tools, textiles, documents and precious treasures. More than that, the relic provides a...