Because the oceans follow the contours of the Earth's gravity field, we can determine its shape using altimetry measurements - or, more directly, by measuring the geoid, which is the shape the ocean surface will tend to take free from other influences such as tides, winds and currents. The gravity field varies over long distances due to variations in the planet's crust, mantle and core. Other variations over smaller distances largely reflect the topography of the ocean floor.
Some of the most striking topographic features are ocean ridges, areas where tectonic plates are moving apart and the ocean floor is constantly forming. Here, we can see a network of faults crossing the ridge, revealing the underlying tectonic motions of the Earth's sphere, as well as the direction of fractures and the speed at which they are opening.