First there came ground mapping radar. This was during World War II. Several decades later we had ground penetrating warfare that could give you a sense of what was underground. Not too far down, but it was useful. Now the US Air Force has developed a sensor that can detect the minute change in gravity created when a tunnel or bunker is hollowed out of the ground. This shows you where the enemy is hiding, the better to bomb those previously unseen and seemingly safe places. The new sensor may be in regular use during the next few years.