Ultrasonic sounds are used to measure the depth of oceans and the principle behind is known as sound navigation and ranging, Sonar. Ultrasonic waves are sent from a transmitter from the surface of the sea, from a ship.
The sound waves are reflected by the sea bottom are sensed be a receiver system and recorded. With the known value of the speed of sound in water, and the time taken by the sound to travel from the surface to the bottom and back, the depth is measured.