Sonar is like underwater radar. It's used in military, also in mapping bodies of water, and it helps fishermen find fish.
Does your question also include open air sensing? Ultrasonic sensing is widely used in range-finding, traffic detection and in robot projects to detect obstacles.
Sonar is used by animals too, with such expertise as to baffle researchers. There is opportunity for breakthroughs to be made.
Dolphins emit sonar clicks toward other creatures, allowing them to 'see' inside their bodies, discern maladies, etc. They locate edible creatures under the sand.
Researchers have only started to decipher how they communicate via sonar clicks.