Subterranean Motion Detectors The various Congresses of the U.S.A. have built fences along our Mexican / American border and these fences caused the 'bad guys' to go underground, i.e. to dig tunnels.  Some of the found tunnels have been very well built with rail lines, ventilation, lighting, drainage, communications, etc.  There are many ways to detect a tunnel, but you have to have the sensors almost directly over the top of the tunnel being built to detect it.  Below are several articles on what to date has been done, and what has worked or failed. The BorderTransportationSystem has a different view of the situation, and has a very different solution that may work.  The system has a central 'T' tower that supports the pipelines, electrical work, and High-Speed rail systems; this central 'T' tower is difficult to scale and can prevent people from crossing the border.  Now, it is proposed that the subterranean tunnel detectors be on a set of rails below the highway level and in the center divider.  The sensors can be mounted on automatic self-powered rail cars that can travel over long section of the system, and thus there would not be 'blind spots', and the 'bad guys' would not be able to figure a tunnel location or depth that would work.   This would be a money saver for our Border Patrol as it would need far less manpower to locate and destroy these illegal tunnels. Index of ... Ground Penetrating Radar Tunnel Detection System TiaLinx Tucker Rowan Seismic / Acoustic Sensor Arrays