According to
Wikipedia, Gravity Gradiometry is the study and measurement of variations in the acceleration due to gravity.
Lockheed has been awarded a contract to develop a sensor that uses this method to map tunnels and other subterranean structures, in order to provide data for attacks, as they published in a press release:
The Defense Advanced Research Projects Agency (DARPA) awarded Lockheed Martina $4.8 million contract to design a sensor system that can locate and identify underground targets by spotting gravity-based effects from an airborne platform.
Under DARPA’s Gravity Anomaly for Tunnel Exposure (GATE) program, Lockheed Martin will develop a prototype sensor and system that can detect, classify, and characterize subterranean threats such as tunnels, bunkers, and caches. The sensor system incorporates a gravity gradiometer, an instrument which measures the tiny variations in the pull of gravity. The GATE sensor will detect those variations to discriminate a man-made void from naturally-occurring features such as topography and geology, yielding a near real-time map of what is underground. -snip-
Hiding WMDs away will be becoming more difficult...
Rattler
