Being able to sense Earth's (extremely strong) magnetic field was never "believed to be impossible." The paper is just about resolving the specific mechanism in

Being able to sense Earth's (extremely strong) magnetic field was never "believed to be impossible." The paper is just about resolving the specific mechanism in birds' bodies that allows them 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_Subsurface_water_and_gravity 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.