The USS San Francisco crash-type problem was discussed some while ago on HN. At the time I questioned how this could happen when one would expect a submarine of that sophistication to be equipped with both sonar and gravity detection.

Detecting changes in gravity is common practice in mining surveys (ores often have higher densities) and geological work, so for me why the sub collided with the seamount remains a mystery.

I cannot believe subs of this class don't have such detection on board so could someone more knowledgeable about the subject than me provide an explanation.