from what i readed in news articles about smuggling tunnels in the US-mexico border and terrorists tunnels in the israel-gaza border it seems that there is no technology that can detect deep(few dozens of meters) underground tunnels and that there is only technology for detecting underground excavation of tunnels. but after a further search in google on things like Ground Penetrating Radar(GPR) i saw many websites of companies that claim to have/sell devices that can detect tunnels 30 meters deep in the ground(and even specify that it is for military purpose) which is the dept of at least some of the tunnels. does 30 meters is the edge for GPR technology in detecting tunnels? or that technology can advance to detect tunnels 50 and 60 meters in the ground?. and what about other technologies? in some news article i heard about the possibility of detecting tunnels with gravimeters, it might be actualy feasible in the future?.