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Way back in WW1 tunnelers on each side used Stethoscopes to detect enemy tunnels and intercept them. They did this quite successfully. Surely today with more sensitive instruments it should be possible to detect digging at much longer distances and depths with even greater accuracy. Then having done so to either drill down to the tunnel to drop in a charge or use some form of ROV digger to intercept it. I adequate resources are used it should be possible to destroy any such tunnel before it is completed.

If the opposition uses machinery to try and dig faster that will increase the noise and risk of detection. If they did slowly by hand it may take longer to detect them but even then it should be possible to find the tunnel before completion.

In low population areas where there is little surface activity to worry about, then why not borrow an idea from naval history. Depth-charge them. Every so often in various locations in desert areas quickly drill a bore hole down to 100 to 200 feet then lower down and detonate a 50 to 100 pound charge hopefully the resultant underground shockwaves will give the tunnel diggers some thing think about.