Abstract-Using sonar to detect submarines was an effective method. Current technologies, however, have made it possible for submarines to thwart standard (active or passive) sonar mechanisms [1]. Thus, finding alternatives to detect submarines becomes important and timely. One viable alternative is to use magnetic or acoustic sensors in close proximity of possible underwater pathways a submarine may pass through. This approach may require deploying large-scale underwater sensor networks to form strong barriers for coastline protection.In this paper we first derive the notion of 3-dimensional stealth distance to measure how far a submarine can travel in a sensor network before being detected by a sensor. Next we prove that there is no strong barrier coverage in a large 3-dimensional fixed emplacement sensor field where sensor locations follow a Poisson point process, i.e., there always exists a path by which an adversary informed of the locations of the sensors can move without being detected. Finally, we describe an energy conserving approach to constructing a strong 3-dimensional barrier using mobile nodes so that intruding submarines cannot pass through without being detected.