What causes a tsunami?

A tsunami is a large wave often called a “tidal wave” even though tides and tsunamis are not related.  Caused by an underwater earthquake or volcano, A tsunamis is a pressure wave that can raise water levels as much as 15 feet.  In the open ocean, a tsunami’s wave may be only a few feet high.  Because the wave’s energy extends from the surface to the bottom, that energy is compressed as the wave approaches shallow water, creating higher, and more life-threatening waves.  Tsunamis usually occur in the Pacific Ocean, but they have caused floods in the Caribbean.  Because they can happen on a clear day and are not related to storms, they can catch many people unaware.