We had a neighbor that put loops of wire under a 600kV line and got enough power to run a water pump for cows. The square law makes that kind of thing pretty low-return.

Here's some calculations on the return [0] (usual caveats of stuff found on the internet). Quite low returns indeed!

[0] https://user.physics.unc.edu/~deardorf/phys25/rwp/exam1rwpso...