Passive sonar picks up bounces. A mountain bounces background noise plus currents flow around it which would show that something was there (currents create temp

Passive sonar picks up bounces. A mountain bounces background noise plus currents flow around it which would show that something was there (currents create temp differences and affect sound waves). They might not have had a clear picture, but they would have known something is there. (Source: a bit of experience)