[hosepipe]

[ you would think that after dealing with north korea's insanely large tunneling projects for decades, the US would have some decent detection methods. ]

They do....
The assumtion that they want to use them... is a bit naieve..
They would already BE being used... They are not being used ON PURPOSE...

[WindOracle]

That is true. We have seismic detectors which can detect a person walking anywhere near it.. it can also detect the much greater vibrations of a tunneling project.