If you can then talk to someone who have been in the Baltic sea on a sub or on a military vessel with sonar.
I tell you this sea is very special with several layers of salt.
Can't remember every word one of our office told us.
A sub can be at one place on a given depth and a vessel can be about 2-4 nm away in a given direction and have it's sonar at the right depth and the ship can't hear the sub. But about 30 NM longer south an another ship hear the sub clearly(it was something like that, it was a lot more we were sitting in a classroom)
Markus
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