former US submariner here: * forward-facing sonar is always being used at all times while a submarine is underway * however, the sonar that's always used is p

former US submariner here: * forward-facing sonar is always being used at all times while a submarine is underway * however, the sonar that's always used is passive, and since mountains don't move, passive sonar doesn't find them * active sonar could theoretically be used, but the risk of communicating a submarine's position relative to the chance that you'd hit an underwater mountain is balanced strongly on the side of running silent.