By common sense, soundwave can be reflected. Based on this principle, sonar is invented. Technically, sonar is a system that utilizes transmitted and reflected underwater sound waves to detect and locate submerged objects or measure the distances underwater. There are two kinds of sonar: One is passive sonar, and the other is active sonar. For passive sonar, we don't send sound waves but just receive them. By listening to these special sound waves, animals that communicate with these particular sound waves can be detected. By listening to the sound waves from engines of submarines or ships, we can locate them. Also, other things that vibrates can be detected by passive sonar. Many threats on the sea are usually detected by passive sonar this way. Since passive sonar does not transmit any thing to the environment, it does no harm to other animals. Active sonar is a little different. Besides having a receiver, active sonar also transmits sound waves. Active sonar works by transmiting particular sound pulse and measuring the length of time for the sound to be reflected back, then devide the time by 2 to get the time it takes for the sound pulse to reach the object. In X-Men: First Class (2011), Banshee is a guy that can transmit ultra sound from his mouth and locate the submarine. I know this is comics and it is fake, but the idea of it is basically active sonar. Even though Banshee is not real, but a bat can do the same thing. Bats are actually blind, but they are able to transmit soundwave and hear them reflect so they can get directions. Active sonar is harmful to some marine mammals like whales. Reason is, these animals' natural sonar get interupted by those sound waves transmitted from active sonar. Sometimes active sonar makes whales surface too early and cause serious decompression. The worst result to them is death.