A mine that either passively listens to a target's sound noises, or periodically interrogates its environment by actively emitting acoustic pulses that may return echoes if prospective targets come within range. A mine is an underwater weapon consisting of a shell case which contains high explosives. Mines can be planted by airplanes, surface ships, or submarines. Submarines are used for deployment when secrecy is important. There are three types of mines: drifting, moored, and bottom. Drifting mines float freely at the sea surface. Moored mines are positively buoyant and are held at a given depth by a cable anchored to the bottom. Bottom mines rest on the sea floor. A mine can be activated by various means, such as by actually contacting a target, by sensing a target's magnetic field, by listening to the acoustic noises that emanate from a target, by sensing the excess pressure field that may be induced on the mine's sensor by a target passing above it, by the reception of acoustic echoes that a target may return to the mine's sonar after it has sent out its interrogating pings, or by a combination of several of these.