Earth’s deep seafloor is, for the most part, a vast, unexplored terrain. Only a miniscule fraction has ever been observed directly, using deep submersibles and remotely operated vehicles. Most of our understanding of seafloor physiography is instead derived from remotely sensed data, such as sonar or satellite altimetry. Sonar data must be collected by ships or underwater vehicles, which is an expensive and time-consuming process. Technological advances over the past two decades have enabled sonar devices to collect detailed “swath” coverage (Figures 1b and 2b), but only a few percent of the seafloor has been mapped in this fashion, typically in areas of prominent seafloor structures such as mid ocean ridges, fracture zones and trenches. More generally, sonar coverage of the ocean floor is limited to profile coverage along disparate ship tracks, well concentrated in shipping lanes and very sparse elsewhere.

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