The ocean is vast, deep, and unexplored. When Malaysia Airlines Flight 370 disappeared three years ago this week, the search brought the ocean’s vastness into sharp relief. This is how deep and dark it is three miles down. This is how unlikely you are to spot a downed airliner in 120,000 square nautical miles of open ocean. This is how much we know about the ocean floor—less than we know about the surface of Mars. This is how a remote part of the Indian Ocean has become, by chance, one of the best-mapped parts of the underwater world.