"Birds Can See Earth's Magnetic Fields, And Now We Know How That's Possible" (https://www.sciencealert.com/birds-see-magnetic-fields-crypt...)
The only reason you can state with confidence that a human with stick can't detect water underground is that they've never been able to do it.
The gravitational variation caused by water deep below the surface is infinitesimal. Its effect on a stick in your hands cannot be felt. You experience far more variation in gravity due to the moon's orbit, and you can't feel that either.
https://www.researchgate.net/publication/272887140_Subsurfac...
> The gravity variation computed from the total water storage (sum of soil moisture and ground water) using the Bouguer approximation was 12 µgal peak-to-peak (1 µgal== 1 x 10-8ms-2).
That's a 0.0000012% difference. You'd experience vastly more difference in force due to the motion of air against the stick as you moved, or the accumulation of dust on the stick over time. Or the local humidity and pressure. Or the position of the moon. Or your elevation. Or your latitude. Or...
Meanwhile, it turns out that if you're over a groundwater deposit, you can dig pretty much anywhere and hit water eventually.
That's a very good reasoning why it is an unlikely thing to be. But the sensitivity of humans to gravitational differences is not a law of physics. This is a very important distinction. The phrase "breaks the laws of physics" is not just a rhetorical flourish.
If I point at a go-kart looking thing and say "that thing can go 0.8 Mach" that's a practical impossibility. For many reasons it is very unlikely, to be true. But does not break the laws of physics. (Who knows, maybe it has a nuke under it, which can loft it that fast for one glorious moment of go-karting fame.)
If I point at the same go-kart looking object and say "that thing can go 1.2c" that is a whole different ballpark. It might still happen, but if it happens that will make us re-evaluate nearly everything we think we know about how the universe works. This is what it means that something "breaks the laws of physics".