You hear those in conventional subs, too, as you go deeper the hull makes all sorts of worrying creaks. (During a sea trial, I had to be aboard a sub from $NAVY

You hear those in conventional subs, too, as you go deeper the hull makes all sorts of worrying creaks. (During a sea trial, I had to be aboard a sub from $NAVY diving to 900 feet and change - the crew assured me the sounds were normal...)