The undersea environment limits the ability of the submarine to accurately sense objects around it. Sound rather than light is the primary sensory medium, but sound transmission underwater is distorted by temperature, pressure, and salinity. In this environment sound bends and refracts; it reflects at odd angles off a rocky bottom or a temperature layer or front; and it is absorbed by the ocean bottom. Sound channels and paths can produce shadow zones where even a noisy contact* can disappear (Urick, 1983). For a number of reasons (including stealth and whale protection), submarines primarily use passive sonar, which cannot directly determine the distance (range) to a sensed object. Relying on passive sonar is rather like relying on vision in a funhouse of distorted mirrors and optical illusions.