Where we don't yet have hyper-accurate bathymetric data there are boats like the USS Jimmy Carter which have lots of tricks and capabilities that we'll never learn about.
Like, of course they do. If there are any unusually-good ocean maps they have, it’s those, at least.
Tapping cables is definitely something that's been done through history, but more of the stories were about things like recovering the remains of enemy test missiles for physical intelligence. Now that that fiber traffic is routinely encrypted anyway, I wouldn't be surprised if more of USS Jimmy Carter's missions are recovering North Korean rockets.
> like is there a big geologic underwater arch somewhere that subs pass under for shits and giggles.
I can't promise there's not, but most naval services have proud traditions of firing captains who are involved in collisions, allisions, and groundings.