Ocean waves are the rhythmic rising and falling of ocean water due to wind or some other disturbing force. They are not a movement of water but instead a movement of energy. There are four types of ocean waves: progressive, internal, standing, and seismic. A progressive wave moves outward from a disturbing force like wind. An internal wave forms from differences in density. A standing wave is the sloshing back and forth of water, and a seismic wave forms from geological events under the sea, such as earthquakes. Those waves that occur in shallow water have different characteristics than those that occur in deep water. No matter the wave type, waves share the basic anatomy that allows them to be measured by crest, trough, height, wavelength, period, and frequency.