Less than a quarter of the ocean floor has been mapped in detail, a fact often cited to argue that we know more about the surface of Mars than about our own planet. The comparison is rhetorically effective but slightly misleading: Mars has no ocean to hide its surface, so mapping it is largely a matter of orbiting cameras, whereas seawater absorbs light and radio waves, forcing surveyors to rely on slow, ship-based sonar. The real obstacle to ocean mapping is therefore not a lack of interest but a difference in physics. New autonomous underwater vehicles, which can survey continuously without a crewed support ship, are expected to close much of the gap within decades.