The other day, I woke up early, flicked on my computer, and dove some 400-plus metres to the bottom of Lake Superior. The largest freshwater lake in the world by surface area, holding enough water to cover all of North and South America under 30 cm of the stuff, Superior is a cold, mythical (cue “The Song of Hiawatha” and “The Wreck of the Edmund Fitzgerald”) and generally mysterious place. And “Superior Maximus”—its deepest point—more mysterious still.