A type of sonar in which you do not send out pings, but just listen for noise made by things under or above the water. The advantage of passive sonar is that no one can hear you using it. Things you can hear with passive sonar include, but are not limited to, surface ships, torpedos, aircraft near the surface of the water, other people's active sonar, the noise of the screws of submarines if they're going fast enough, someone dropping a wrench on the deck of a sub, hull-popping noises from a sub changing depth, whales, and other such things. If you can hear something with sonar, you can locate it. If you can locate it, you can kill it. Making subs more and more quiet is an ongoing task of engineers.