Remember how the "sensors" on scifi craft like the starship Enterprise could detect almost anything on a planet, even underground or inside buildings? DARPA is investigating two approaches to provide something close to those abilities. One would use airborne gravitometers to measure microchanges in gravity created by subsurface voids (bunkers or tunnels). The other would use radar signals bounced off buildings to "see" into the concrete canyons of cities, where normal radar can't detect anything but the buildings themselves, to track vehicles.
COMMENT: Tough challenges, but DARPA would say that's what the agency is for. Both would have immediate impact in war zones, and the tunnel detection has other uses, from nuclear inspection to drug smuggling. Good luck, chaps.