Early Mapping.

With the development of sonar in World War II, a means was provided for studying the contour of the land underneath the oceans. Before sonar, the configuration of the ocean floor was mapped by a crude tracing method. A weight on a long chain or cable was lowered from a ship until the weight hit bottom. The depth of water at that point was determined. Then the weight was dragged slowly to show how the depths changed from place to place. Very deep oceans, strong currents, and turbulent seas limited the usefulness of this method.

Sonar.

With sonar the picture is much clearer. As a ship moves along it can bounce sound waves down to the ocean floor. The reflection of the sound waves indicates the depth of the water and the shape of the bottom. Ships using sonar can...