Yep, just sound. And you don't even know range, you only know how strong sound is on a particular bearing. The Navy trains to do a series of maneuvers that he

Yep, just sound. And you don't even know range, you only know how strong sound is on a particular bearing. The Navy trains to do a series of maneuvers that helps you understand the range to the ships you do hear, and positions the sub most safely to come to periscope depth. And then while you're coming to periscope depth you're looking out to see if there are any indications you've screwed up. But yeah, there's nothing visual available when you're under the water.