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Bathymetry is the study of the underwater depth of sea and ocean floors, lake floors, and river floors. It has been carried out for more than 3000 years, with the first recorded evidence of measurements of water depth occurring in ancient Egypt. Bathymetric measurements are conducted with various methods, from depth sounding, sonar and lidar techniques, to buoys and satellite altimetry. However, despite modern computer-based research, the seabed remains less well measured in many locations than the topography of Mars. Bathymetry has various uses, including the production of bathymetric charts to guide vessels and identify underwater hazards, the study of marine life near the bottom of bodies of water, coastline analysis, and ocean dynamics, including predicting currents and tides. This video, created by the Scientific Visualization Studio at NASA's Goddard Space Flight Center, simulates the effect on a satellite map of the world of a gradual decrease in worldwide sea levels. As the sea level drops, more seabed is exposed in shades of brown, producing a bathymetric map of the world. Continental shelves appear mostly by a depth of 140 metres (460 ft), mid-ocean ridges by 3,000 metres (9,800 ft), and oceanic trenches at depths beyond 6,000 metres (20,000 ft). The video ends at a depth of 10,190 metres (33,430 ft) below sea level – the approximate depth of the Challenger Deep, the deepest known point of the seabed.Animation credit: NASA / Goddard Space Flight Center / Horace Mitchell, and James O'Donoghue