Gravity gradiometers in use today have their origins in naval special warfare because they ensure safe navigation and accurate detection of underwater objects. Submarines rely on accurate detection of their surroundings, and gravity gradiometry is a passive technique that does not compromise the submarine's position because it is a technique that does not emit energy. However, the way submarines achieve collision avoidance has never been addressed. Using gravity gradiometry data in real time warns of the existence of an underwater mountain in a submarine's trajectory. The method does not require accurate positioning, extensive measuring over large areas, or a gravity-gradient map of the ocean bottom. The results of a study demonstrate how gravity gradiometry can be used effectively to detect anomalous mass in a trajectory quickly, without involving sophisticated processing or interpretation algorithms. For many years, geophysicists have known the original use of the modern gravity-gradiometry technique without going into the details.