by David Murphy
In the open ocean, tsunamis are often only several feet high and many miles long. In fact, they can go completely unnoticed by mariners as the wave passes beneath their boats.
In 1896, a tsunami erupted off the coast of Japan. Fishermen in their boats were completely unaware of the tsunami as it passed beneath their fleet. They only learned of it when they came back to port at the end of the day---and discovered their fishing villages destroyed.
A tsunami, upon reaching shore, grows dramatically in height as the energy in the wave forces more and more water up and over the shoreline. A tsunami's height as it contacts a beach can be 20 to 50 feet high. If the tsunami is channeled into a narrow bay or harbor, surrounded by hills, the water can reach as high as 100 feet. This is a particular problem when tsunamis come ashore in mountainous sections of Japan.