That first map was only of the gravity field but to anyone who had ever seen a map of seafloor topography, its broad outlines looked familiar. That should not be surprising: insofar as mountains tend to have more mass than valleys, topography generates gravity, “If you put a mountain on the seafloor,” explains Smith, “the extra material represented by the rocks in that mountain add their ovm gravity to the overall field. If you’re right above the mountain, the added gravity pulls down in the same direction, and so it adds to the magnitude of gravity. But if you’re off to one side of the mountain, the gravitational field of the mountain pulls toward the mountain, and so the effect is to change the direction of gravity 3 little bit.” The sea surface, acting as a carpenter’s level, follows these changes; it becomes like an attenuated visual echo of the seafloor, piling up over mountains, dipping down over trenches.
Mapping The Deep - Robert Kunzig