An acoustic mine is a type of naval mine which monitors audio activity in its vicinity. Depending on its design, it will either actively send out audio pulses, not unlike a sonar, listening to the speed at which the echo returns to it or passively listen to its environment, depending only on the noise that is made without its interference. Further variants exist, as this mine may either freely drift, be moored at a certain location and depth, or lie at the bottom of the ocean floor.