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Three fifths of the Earth's surface is under the ocean, and the ocean floor is as rich in detail as the land surface with which we are familiar. This animation simulates a drop in sea level that gradually reveals this detail. As the sea level drops, the continental shelves appear immediately. They are mostly visible by a depth of 140 meters, except for the Arctic and Antarctic regions, where the shelves are deeper. The mid-ocean ridges start to appear at a depth of 2000 to 3000 meters. By 6000 meters, most of the ocean is drained except for the deep ocean trenches, the deepest of which is the Marianas Trench at a depth of 10,911 meters. Credit for files used: Lead animator: Horace Mitchell (NASA) NASA/Goddard Space Flight Center Scientific Visualization Studio U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Geophysical Data Center, 2006, 2-minute Gridded Global Relief Data (ETOPO2v2) - http://www.ngdc.noaa.gov/mgg/fliers/0... The Blue Marble Next Generation data is courtesy of Reto Stockli (NASA/GSFC) and NASA's Earth Observatory. The Blue Marble data is courtesy of Reto Stockli (NASA/GSFC).

5.1 Mapping the Topography of the Ocean Floor; Underwater Volcanoes, Mountains and Valleys The ocean is the largest feature on Earth, covering more than 70% of our planet's surface. One of the main reasons that Wegener's continental drift hypothesis was not widely accepted when first proposed was that so little was known about the ocean floor. Until the 20th century, investigators used weighted lines to measure water depth. In deep water these depth measurements, or soundings, took hours to perform and could be wildly inaccurate. With the development of new marine tools following World War II, our knowledge of the diverse topography of the ocean floor grew rapidly. One of the most interesting discoveries was the global oceanic ridge system. This broad elevated landform which stands two to three kilometers above the adjacent deep ocean basins, is the longest topographic feature on Earth. Today we know that oceanic ridges mark divergent plate margins, where new oceanic lithosphere originates. We also know that deep ocean trenches represent convergent plate boundaries, where oceanic lithosphere is subducted into the mantle. Because the process of plate tectonics is creating oceanic crust at mid-ocean ridges and consuming it at subduction zones, the oceanic crust is continually being renewed and recycled.