After you complete getting the message out these guys contact you - so after you make your way back to town and re-supply head back to the Sewer entrance and follow the marker and make your way back to the area where the Antennae was - and past it.
Eventually you come to the cave in which the brothers Grimm are working on their submarine?!
Speak to Volkan Dal there - he seems to know of you. OK remember that Sonar Unit? Remember those two geek-jerks? Yeah this is the culmination of that earlier side-quest set!
Well that went pretty good! The sub sank! Nice one. And that pretty much completes this side-quest!