We all know what gravity is, but to understand how it is used in mapping the ocean floor, let us look at it in slightly greater depth. The story of Sir Isaac Newton and the apple is well known. From this, he formulated the Law of Gravity, which states that the force between two bodies is a function of the masses and the distance between the bodies. This equation simply means that heavier bodies exert greater gravitational force, and the smaller the distance between them, the greater the force. On Earth, there are several factors to be considered for these variables.
Converse is true for low elevations.
Topography of surrounding area. If a mountain is nearby, it exerts its own gravitational pull because of its extra mass. If there is a valley, there is reduced gravitational pull because of lack of mass. Density - directly affects the mass of a feature. For example, basalt is denser than andesite.