DHS tries to find tunnels below the surface

Federal Times: The surface of the earth has been mapped and mastered, with lidar, photography, infrared and other now-familiar systems, on satellites, manned aircraft and drones. Thermal imagers can even penetrate thin layers of soil. But go just a bit deeper, and you reach one of the last frontiers of ISR. How do you see into solid ground, below the roads, the rocks and the desert sand?

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