It would be a waste of time to drill into rock for an RF ground, unless
the rock was iron ore, or silver ore!
What hams need to do with this case, which is very common, sometimes not
even rock but caliche like I have, or other poorly conducting soil, is
to lay out quarter wave radials or a counterpoise wire at least. Run
the radials in all directions you can like a crow's foot. This is also
called a crow's foot ground.
You might zigzag some wire in that low crawl space, to fit the area.
Just make sure you also bond the radio ground(s) to the AC electric
safety ground to avoid setting up a difference of potential in case of a
strike on your antenna.
I would unplug all my radios from feedlines, and AC in such a poor
ground environment, when not in use and/or you are not home.
The best surge suppressor is not going to help much in case of a direct
strike, and at your location the lightning has no easy path to take.
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