It’s not free at all. Most of the voltage drop along the cable is caused by conversion of electrical energy into photons within the erbium-doped fiber amplifiers. A relatively small fraction of the voltage drop is caused by losses in the copper cable that carries the current along the route. The high supply voltage allows a relatively small amount of current to carry thousands of kilowatts of power to the amplifiers without causing much loss in the copper.
I took that as referring to how over large distances the results of driving a metal rod into the dirt don't always match, so if you do things like tie both ends of a shielded cable's shielding to separate ground rods you can get odd problems sometimes.
Although I hadn't thought the differences were usually anywhere close to that large.
You’re referring to creating an intentional ground loop, I believe [https://en.m.wikipedia.org/wiki/Ground_loop_(electricity)].
The challenge as I understand it, is that yes you will get ‘free’ power (not actually free, as you had to create the low resistance electrical path for it to exist), but you have no control over the properties or values of what you get - and it will vary unpredictably.
It’s also unlikely you’ll consistently get much actual net power out of it, as you’re competing against an entire planets worth of reasonably conductive (in bulk) parallel paths.
It’s almost always a problem because of that.
It’s almost always a problem because of that.