A system for passively monitoring territory proximate to or at restrictive boundaries for tunnels, the system comprising a plurality of muon sensors, a data network in communication with each muon sensor, a power network in electrical communication with each muon sensor, and a data analysis unit, the data analysis unit in communication with each muon sensor via the data network, the data analysis unit comprising a memory and a processor, the memory configured to instruct the processor to analyse data from the plurality of muon detectors to identify and locate a new or emerging tunnel. A method of locating tunnels is also provided.