Passive sonar shows "things making noise". It doesn't show things that do not make noise.

Mountains do not, as a general rule, make noise, and do not show up on passive sonar.

In addition, when running at flank speed the efficiency of passive sonar is greatly degraded by flow and machinery noises.

The way a submarine avoids running into seamounts is a combination of very good maps, very good dead reckoning, (IIRC) gravitational anomaly detection (tied to more great maps), and if all else fails high-frequency (short-range) active sonar. It's a really interesting system! But I don't think passive sonar ties into it.

I'm curious why this reply got downvoted so much. Can someone explain if the content is correct or not and why?

I've got no clue about the downvotes either. Maybe because the first version of the comment was a little snarky, and didn't include how submarines actually avoid crashing into things?

> very good maps, very good dead reckoning

There's a whole series of highly classified inertial navigation units used in aviation and submarine applications. The ones for use above the ocean's surface have many DoD applications in a theoretical GPS-jammed environment in warfare against an equal-tech adversary.

Yeah, I should have said "very good inertial measurement systems" instead of dead reckoning. It doesn't leave quite the same impression!

Mountains do make noise when they reflect the sound your prop makes. I took the gp to refer to that. Whether you can use that for collision avoidance, no idea.

There's a few problems with this. For one, your prop noise is going to be almost entirely masked directly ahead of you, because there's a whole submarine in the way. Isolating "sound of our propellers from in front of us" from "our propellers behind us" is certain to be extremely difficult if it's not completely impossible too.

The sound of the water rushing pass your bow would deafen any passive sonar arrays when operating at flank speed…