Clandestine tunnels continue to pose an invisible threat to secure facilities and national borders despite decades of work and resources expended by multiple governments. Tunnels are a low-tech solution to a more formidable and advanced adversary, requiring little more than time, patience, and manpower. If there is one thing that has been learned through previous work, it is that finding tunnels is a difficult and complex problem that does not have a simple solution. There is no “silver bullet” that will work in all geologic and cultural settings with a 100% success rate and a 0% false-positive rate. Despite the challenges, notable progress has been made in recent years through advancements that use near-surface seismic methods to detect tunnels.