In the summer of 2010, Christian Ekström, a diver from the Åland Islands, an autonomous region of around sixty-five hundred isles off of Finland’s west coast, began searching for a shipwreck in the Baltic Sea, based on a tip he’d received from a fisherman. The Baltic’s temperature is unusually consistent (between about thirty-nine and forty-three degrees Fahrenheit* on its seabed), and it has a salinity level that is less than a fifth that of oceans. Its coastal waters are also treacherously shallow. All of this makes it particularly well suited to sinking ships, and then, once they’ve sunk, to preserving them for centuries. (Creatures commonly known to erode wrecks, like shipworms, can’t survive in such brackish waters.) As a result, the Baltic has an estimated hundred thousand shipwrecks, only a fraction of which have been explored.