Read the following text. Answer the questions based on the text.

    A tsunami is one of the most powerful and destructive natural forces. It is a series of extremely long ocean waves started by a sudden movement of ocean water, usually by an earthquake.

    Although most tsunamis are generated by earthquakes on adjoining tectonic plate boundaries, they can also be caused by volcanic eruptions, landslides, undersea slumps, or even meteorites. Tsunamis radiate outward in all directions from the point of origin and can move across entire ocean basins. When they reach the coast, they can cause dangerous coastal flooding and powerful currents that can last for several hours or days.

    Tsunamis are also known as seismic sea waves, when they are caused by earthquakes. An earthquake must be big enough and close enough to the ocean floor to cause the vertical movement of the ocean floor that typically sets a tsunami in motion. As the ocean floor rises or drops, so too does the water above it. As the water moves up and down, seeking to regain its balance, the tsunami radiates out in all directions.

    Tsunamis can also occur when a landslide enters the water and moves it from above or when water is moved ahead of and behind an underwater landslide. The tsunami generated by a landslide depends on the amount of landslide material that moves the water, the speed it is moving at, and the depth it reaches.

    Volcanic eruptions too, although rarely, can cause tsunamis. Several types of volcanic activity can move enough water to generate destructive tsunamis. Like other non-seismic tsunamis, such as those generated by landslides, volcanic tsunamis usually lose energy quickly and rarely affect distant coastlines.

(Adapted from: www.tsunami.gov, www.noaa.gov)

The subject of the first sentence in the first paragraph is ____