[SmithL]

Unless it was a mighty noisy mountain, passive sonar never would have heard it.

[Goodgirlinred]

Then how was he to have known it was there?

[wideminded]

Unless it was a mighty noisy mountain, passive sonar never would have heard it.

I have heard that a type of sonar has been invented that uses ambient sound to visualize objects underwater, much as one can visualize objects on the surface with indirect, ambient light that comes neither from the observer nor the target. Apparently, this new type of sonar is not yet used on submarines.