Apparently when doing saturation diving like that you can’t even understand what the other person says, between the helium and the pressure the voice is too distorted to be intelligible.
You can communicate with a computer that transforms your voice to be intelligible but it is really not a pleasant conversation so you can stay there for weeks without having a conversation except for the bare minimum.
There was a huge network of pneumatic tubes under Paris used by the postal services between 1866 and 1980.
With that you could send a letter to any address in the city that would arrive in less than 2 hours.
https://youtu.be/Ns5SKH-ooy0