Nothing's going through a kilometer of rock, but if you have a few meters of blocked tunnel, neutrons will go through that just fine. If you actually had a kilo

Nothing's going through a kilometer of rock, but if you have a few meters of blocked tunnel, neutrons will go through that just fine. If you actually had a kilometer of rock between the miners and help, the miners are dead. https://en.wikipedia.org/wiki/Gauss%27s_law_for_magnetism https://en.wikipedia.org/wiki/Gauss%27s_law_for_magnetism Flipping a dipole is literally just radio. How do you think nuclear reactors are controlled? Nuclear reactors can vary their power output at a rate of tens or even hundreds of MW/minute. While it takes special designs to throttle at high enough speeds for load following, to send a signal you only need to change the power output enough to be detectable by a neutron detector, which need only be slightly greater than the variation due to random noise.