The term 'Periodically Rising and Falling' can be summarized as TIDAL, which is a five letter word. TIDAL refers to the regular rise and fall of the sea level due to the gravitational pull of the moon and sun. This phenomenon is caused by the gravitational attraction between the Earth and the moon, which causes the ocean's surface to bulge out in the direction of the moon. The bulging of the ocean's surface creates high tides, while the opposite side of the Earth experiences low tides.