I purchased a hydrophone (a microphone for listening to underwater sound) several years ago so my family and I could listen in on the killer whales when they went past our kayaks; most of the time, in this way, we could hear them even from considerable distances. The sounds that we have heard over the years have always seemingly involved members of the pods communicating information to each other; the call and response interactions were clear, and the variety of the sounds made it obvious that this was not just random noisemaking.
