Single wire, earth-return phone circuit diagram

Hey guys,
Not lights related (hence in the off topic forum ) but since many of you have electronics knowledge I thought maybe I can find some help here.
I want to build a single wire, earth-return phone for long distance (1000s of meters) for using in cave rescue operations.
All circuit diagrams available online are in Russian or very old so some components are hard to find.

If you have any source for update circuit diagram please share

I don’t have a reference handy, but I bet a military 2 wire field phone reference might have what you need.

Military 2 wire field phone is a good solution but I am looking for a single wire circuit in order to save half of the wire required

you misunderstand - two wire phones are the simplest, and if any can, they are the mostly likely to be able to use a common earth ground.

He’s asking to use the earth itself as the second wire. I don’t think the audio quality will be acceptable in a life-or-death rescue.

This method is very common among cave rescue teams.
Double wire or wireless systems also have cons :( .
Unfortunately commercial systems are no longer available but many rescue teams build their own systems