I think old-time submariners pounded metal on metal with tools agaianst steel hulls to make signal noises. How would that work with a carbon fiber hull? Could they find heavy enough metal object to clang together to bea heard outside the ship by potential rescuers?
And why on Earth would they apparently not have an ocean locator beacon or transmitter to turn on to aid in rescue, like the Epirbs that boaters can and should buy? Water activated and auto matic. SMH....
Not a submariner or anything close to it - but communicating with subs, even at a deep depth of 1000 feet, needs very low frequency radio waves. I expect its a speciality, very costly system with limited commercial/public practicality.
and this thing is at 13,000 feet.
Even a balloon would suffice...................
The front dome of the sub is titanium...............