Most of the ocean floor has never been mapped in detail. Autonomous underwater and surface robots, carrying sonar and sensors, survey the seabed over long missions, charting its shape and features far more efficiently than crewed ships alone.
Better maps support science, safe navigation, cable and pipeline routing and understanding of marine environments. The robots must endure long missions, pressure and the difficulty of communicating underwater, and the ocean is vast, so mapping it is a long effort, but autonomous craft are steadily filling in one of the largest remaining blanks on the map.